Water-absorbing agent

ABSTRACT

There is disclosed a water-absorbing agent which combines both performances of the capillary suction force and the liquid permeability. This water-absorbing agent is a particulate water-absorbing agent comprising water-absorbent resin particles (α) and a liquid-permeability-enhancing agent (β), wherein the water-absorbent resin particles (α) are surface-crosslink-treated particles of a crosslinked polymer of a monomer including acrylic acid and/or its salt; with the water-absorbing agent being characterized in that the particulate water-absorbing agent has: a mass-average particle diameter (D50) in the range of 234 to 394 gm, a logarithmic standard deviation (σζ) of a particle diameter distribution in the range of 0.25 to 0.45, an absorption capacity without load (CRC) of not less than 15 g/g, and a water-extractable component content of not higher than 15 mass %; and further a liquid-permeability-enhancing agent (β) content in the range of 0.01 to 5 mass parts per 100 mass parts of the water-absorbent resin particles (α).

TECHNICAL FIELD

The present invention relates to a water-absorbing agent. Morespecifically, the present invention relates to a water-absorbing agentwhich combines excellent capillary suction force and liquid permeabilitywherein the water-absorbing agent is obtained by: modifying surfaces of,water-absorbent resin particles (α) with a crosslinking agent, whereinthe water-absorbent resin particles (α) are regulated to a specificmass-average particle diameter and a specific particle diameterdistribution; and containing a liquid-permeability-enhancing agent (β).

BACKGROUND ART

At present, water-absorbent resins (water-absorbing agents) andhydrophilic fibers (e.g. pulp) are widely used for sanitary materialssuch as disposable diapers, sanitary napkins, and so-called incontinentpads as their component materials for the purpose of causing thewater-absorbent resins and the hydrophilic fibers to absorb body fluids.Examples of materials used as main raw materials for the abovewater-absorbent resins include: partially-neutralized and crosslinkedpoly(acrylic acids); hydrolyzed graft polymers of starch-acrylic acid;saponified copolymers of vinyl acetate-acrylic acid ester; hydrolyzedcopolymers of acrylonitrile or acrylamide, or crosslinked polymers ofthese hydrolyzed copolymers; and crosslinked polymers of cationicmonomers.

In recent years, as to these sanitary materials such as disposablediapers and sanitary napkins, their high functionalization and thinningare making progress, so there is a tendency toward increases in theamount of the water-absorbent resin as used per piece of sanitarymaterial and in mass % of the water-absorbent resin relative to a wholeabsorbent structure consisting of such as the water-absorbent resin andthe hydrophilic fibers. Specifically, the ratio of the water-absorbentresin in the absorbent structure is raised by decreasing the amount ofthe hydrophilic fibers (which have a small bulk density) and increasingthe amount of the water-absorbent resin (which has excellent waterabsorbency and a large bulk density) as used. Thereby the thinning ofthe sanitary materials is aimed at without lowering the water absorptionquantity.

(1) Liquid Permeability and Liquid Diffusibility:

However, the sanitary materials, in which the ratio of the hydrophilicfibers has been decreased and that of the water-absorbent resin has beenincreased in the above way, are in a favorable direction from theviewpoint of simple storage of liquids, but rather involve problems inthe case of consideration of distribution and diffusion of the liquidsunder circumstances of actual use of diapers. The large amount ofwater-absorbent resin swells to become a soft gel due to waterabsorption to cause a phenomenon which is called “gel-blocking” thatmuch hinders the permeation and diffusion of the liquids. Known examplesof arts of improving these liquid permeability and liquid diffusibilityinclude the following arts.

There is known a method in which there is used a hydrogel-formableabsorbent polymer of which: the saline flow conductivity (SFC) value isat least about 30 (10⁻⁷·cm³·s·g⁻¹); the capacity value of performanceunder pressure (PUP) is at least 23 g/g under a closing pressure of 0.7psi (5 kPa); and the basis weight is at least about 10 gsm (patentdocument 1).

There is known an absorbent structure of which: the water-absorbentresin concentration is at least 40 mass %; the saline flow conductivity(SFC) value is at least about 30 (10⁻⁷·cm³·s·g⁻¹); and the capacityvalue of performance under pressure (PUP) is at least 23 g/g under aclosing pressure of 0.7 psi (5 kPa) (patent document 2).

There is known a method in which: in an absorbent structure, awater-absorbent resin of which the gel layer permeability (GLP) is atleast 4 (10⁻⁷·g⁻¹) is used for an upper layer, and a water-absorbentresin of which the absorption capacity under load (AAP) is at least 15(g/g) under a load of 50 g/cm² is used for a lower layer (patentdocument 3).

There is known a method in which a polycation is covalently bonded to awater-absorbent resin (patent document 4).

There is known an absorbent material including a mixture of: a pluralityof absorbent gel-formable particles including a water-insoluble andwater-swellable polymer; and an absorbency-improving polymer which isreactable with at least one component contained in urine (patentdocument 5).

There is known a method in which there is used a mixture of a sphericalwater-absorbent resin and a non-spherical water-absorbent resin (patentdocument 6).

There is known a method in which there is used a water-absorbent resinof which the saline flow conductivity (SFC) value is at least 5(10⁻⁷·cm³·s·g⁻¹) and which contains a permeability-keeping agent (patentdocument 7).

There is known an absorbent structure having a division surrounded by acontinuous area of a hydrogel absorbent polymer (patent document 8).

There is known a hydrogel-formable absorbent polymer of which: thedynamic gelation rate is at least about 0.18 g/g/sec; and the abilityvalue of performance under pressure (PUP) is at least about 25 g/g undera binding pressure of 0.7 psi (5 kPa); wherein the hydrogel-formableabsorbent polymer has a mass-median particle size of at least about 100μm when the hydrogel-formable absorbent polymer exists in the form ofparticles (patent document 9).

There is known a water-absorbent material, in the rear half portion ofwhich there is placed 55 to 100%, favorably 60 to 90%, of the entiremass of an absorbent gelling material (patent document 10).

There is known an absorbent member having a liquid-acquiring zone(patent document 11).

There is known a water-absorbent resin of which: the absorption capacityunder load is not less than 30 g/g; and the gel layer liquid permeationrate is not more than 100 seconds (patent document 12).

There is known a process including the step of grindingcrosslink-polymerized particles until their bulk density increases tonot less than 0.72 g/ml (patent document 13).

There is known a process in which a water-absorbent resin issurface-treated with a surface-treating agent including a polyol and acation (patent document 14).

There is known a process in which a water-absorbent resin issurface-treated with a surface-treating agent including an organiccrosslinking compound (except polyols) and a cation (patent document15).

There is known a water-swellable polymer as crosslinked with anunsaturated amino alcohol (patent document 16).

There is known a hydrogel-formable polymer of which: the saline flowconductivity (SFC) is at least 40 (10⁻⁷·cm³·s·g⁻¹); the AUL is at least20 g/g under 0.7 psi (4826.5 Pa); and the Frangibility Index (FI) is atleast 60% (patent document 17).

There is known a water-insoluble and water-swellable hydrogel which iscoated with steric or electrostatic spacers and of which: the AUL is atleast 20 g/g under 0.7 psi; and the gel strength is at least 1,600 Pa(patent document 18).

However, in cases where the above prior water-absorbent resins asdisclosed in patent documents 1 to 18 have high liquid permeability,spaces between swollen gel particles increase to bring aboutdeterioration of the capillary suction force. The deterioration of thecapillary suction force is a cause that a residual liquid remaining nottaken into the water-absorbent resin increases on surface layers of thesanitary materials to thus bring about: deterioration of the dry-touchproperty; an unpleasant feeling during wearing; and skin diseases suchas skin eruption. In order to avoid such problems to maintain absorptionproperties of the absorbent structure, the ratios of the hydrophilicfibers and the water-absorbent resin are axiomatically limited, so alimit occurs also to the thinning of the sanitary materials.

That is to say, in the prior arts, the liquid permeability is pursued,but, to the capillary suction force as lost thereby, there has been paidno attention. In addition, in the prior arts, though the particlediameter distribution is a very important factor for the liquidpermeation and the capillary suction force, yet, about it, there hasbeen made no detailed explanation of the relationship of the particlediameter distribution with the liquid permeability and the capillarysuction force. As matters now stand, almost no detailed explanation ismade particularly also about a particle diameter distribution which isexcellent in both of the liquid permeability and the capillary suctionforce or about a means for achieving such a particle diameterdistribution.

(2) Particle Diameters:

In addition, such as the following arts are known as examples ofwater-absorbent resins having a controlled particle diameterdistribution and arts in which particle diameter distributions ofwater-absorbent resins are controlled.

There is known an absorbent article including a high-molecular gellingagent having a mass-median particle diameter of 400 to 700 μm (patentdocument 19).

There is known a hydrogel polymer of which: the average particlediameter is in the range of 100 to 600 μm; and the logarithmic standarddeviation σζ of the particle diameter distribution is not more than 0.35(patent document 20).

There are known polymer material particles which are water-insoluble,absorbent, and formable into a hydrogel, and of which at least 80% areparticles of such a size as passes through a sieve having a mesh openingsize of 297 μm and is captured on a sieve having a mesh opening size of105 μm (patent document 21).

There are known water-absorbent resin particles which have a specificsurface area of 50 to 450 m²/g and contain a fine powder of hydrophilicsilicon dioxide having a hydrophilicity of not less than 70% (patentdocument 22).

However, as to the above arts cited in such as patent documents 19 to22, none of them is an art for achieving a particle diameterdistribution specified for obtaining a water-absorbing agent which isexcellent in both of the liquid permeability and the capillary suctionforce. In addition, the ranges of the resultant particle diameterdistributions are also broad, and the absorption capacity without loadand the absorption capacity under load are also different. Therefore, ithas been difficult to obtain from the above arts the water-absorbingagent which is excellent in both of the liquid permeability and thecapillary suction force.

[Patent document 1] WO 95/26209

[Patent document 2] EP 0951913

[Patent document 3] EP 0640330

[Patent document 4] WO 97/12575

[Patent document 5] WO 95/22356

[Patent document 6] WO 98/06364

[Patent document 7] WO 2001/066056

[Patent document 8] WO 97/25013

[Patent document 9] WO 98/47454

[Patent document 10] WO 96/01608

[Patent document 11] WO 97/34558

[Patent document 12] JP-A-089527/2001 (Kokai)

[Patent document 13] EP 1029886

[Patent document 14] WO 2000/53644

[Patent document 15] WO 2000/53664

[Patent document 16] U.S. Pat. No. 6,087,450

[Patent document 17] U.S. Pat. No. 6,414,214

[Patent document 18] US 2002/128618

[Patent document 19] U.S. Pat. No. 5,051,259

[Patent document 20] EP 0349240

[Patent document 21] EP 0579764

[Patent document 22] EP 0629411

As to the aforementioned prior art water-absorbent resins and/orwater-absorbing agents of such as patent documents 1 to 22, the liquidpermeability is improved, but, at the same time, the performancedeterioration such as capillary suction force deterioration is caused.Thereby, the diffusibility and the liquid permeability in awater-absorbent structure which is a component material for such assanitary materials are improved, but, on the other hand, there is causedthe performance deterioration such that the dryness property and theliquid-retaining ability are deteriorated. Therefore, the aforementionedprior art ones have not necessarily been satisfactory ones. That is tosay, there have occurred problems such that, even if either one of theperformances is enhanced, the other performance is deteriorated. Inorder to solve such problems, it has been expected that there appears awater-absorbing agent which combines both performances of the liquidpermeability and the capillary suction force.

DISCLOSURE OF THE INVENTION Object of the Invention

That is to say, an object of the present invention is to provide awater-absorbing agent which combines both performances of the capillarysuction force and the liquid permeability.

SUMMARY OF THE INVENTION

The present inventors diligently studied to solve the aforementionedproblems. As a result, they have found out that the following six pointsare important for achieving the aforementioned object.

(1) The mass-average particle diameter is regulated to around 300 μm,and the particle diameter distribution denoted by the logarithmicstandard deviation of the particle diameter distribution is controlledin a specific range.

The liquid permeability and the capillary suction force have propertiescontrary to each other, and their respective performances change greatlyat around 300 μm as a boundary. Therefore, the water-absorbing agentwhich combines both performances of the liquid permeability and thecapillary suction force can be obtained by specifying the mass-averageparticle diameter and the logarithmic standard deviation of the particlediameter distribution in the aforementioned ranges.

(2) The absorption capacity without load is not less than 15 g/g(favorably in the range of 15 to 33 g/g (but not including 33 g/g), morefavorably in the range of 17 to 31 g/g (but not including 31 g/g), stillmore favorably in the range of 19 to 29 g/g (but not including 29 g/g),most favorably in the range of 23 to 28 g/g (but not including 28 g/g)).

In the case where the CRC is less than 15 g/g, the absorption capacitywithout load is too low and therefore unfavorable for practical use. Inaddition, particularly when the CRC is in the range of less than 33 g/g(favorably less than 29 g/g), the liquid-permeability-enhancing agent(β) remarkably takes effect.

(3) The water-extractable component content is not higher than 15 mass%.

In the case where the water-extractable component content is higher than15 mass % in the present invention, there is a possibility not only thatno effects of the present invention may be obtained, but also that theperformance may be deteriorated in the use for water-absorbentstructures. In addition, such a water-extractable component content isunfavorable also from the viewpoint of safety. As a cause of theperformance deterioration, it can be cited that, when thewater-absorbing agent absorbs water to swell, a high-molecular componentelutes from the inside of the water-absorbing agent to thereby hinderthe liquid permeation.

(4) The liquid permeability is enhanced by containing theliquid-permeability-enhancing agent (β).

Because the liquid permeability is enhanced by containing theliquid-permeability-enhancing agent (β), the water-absorbing agent whichcombines both performances of the liquid permeability and the capillarysuction force can be obtained.

(5) Surfaces of water-absorbent resin particles are crosslink-treated.

In the case where surfaces of water-absorbent resin particles are notcrosslink-treated, there is a possibility that the liquid permeabilityand the capillary suction force may greatly be damaged.

(6) There is possessed the shape of irregularly pulverized particleshaving a larger surface area than such as spherical shape.

By shaping the irregularly pulverized particles, the capillary suctionforce becomes higher in performance, so that the water-absorbing agentwhich combines both performances of the liquid permeability and thecapillary suction force can be obtained.

By satisfying these conditions, the water-absorbing agent which combinesboth performances of the capillary suction force and the liquidpermeability can be obtained wherein such a water-absorbing agent hashitherto never been.

In addition, it is favorable that at least a portion of water-absorbentresin particles (α) included in the water-absorbing agent have a porousstructure.

That is to say, a water-absorbing agent according to the presentinvention has the following constitution.

A water-absorbing agent, which is a particulate water-absorbing agentcomprising water-absorbent resin particles (α) and aliquid-permeability-enhancing agent (β), wherein the water-absorbentresin particles (α) are further-surface-crosslink-treatedirregular-shaped pulverized particles of a crosslinked polymer of amonomer including acrylic acid and/or its salt;

wherein the particulate water-absorbing agent has:

a mass-average particle diameter (D50) in the range of 234 to 394 μm, alogarithmic standard deviation (σζ) of a particle diameter distributionin the range of 0.25 to 0.45, an absorption capacity without load (CRC)of not less than 15 g/g, and a water-extractable component content ofnot higher than 15 mass %; and further

a liquid-permeability-enhancing agent (β) content in the range of 0.01to 5 mass parts per 100 mass parts of the water-absorbent resinparticles (α).

EFFECTS OF THE INVENTION

According to the present invention, the water-absorbing agent whichcombines both performances of the liquid permeability and the capillarysuction force (such a water-absorbing agent has hitherto never been) canbe provided by: surface-crosslinking the water-absorbent resin particlesto such a degree that they will display the specific absorptioncapacity, wherein the water-absorbent resin particles have the specificmass-average particle diameter and the specific logarithmic standarddeviation of the particle diameter distribution; and containing theliquid-permeability-enhancing agent (β).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a measurement apparatus as usedfor measuring the absorption capacity under load (AAP) for the 0.90 mass% physiological saline solution under a load of 4.83 kPa in 60 minutes.

FIG. 2 is a schematic sectional view of a measurement apparatus as usedfor measuring the saline flow conductivity (SFC) for the 0.69 mass %physiological saline solution.

FIG. 3 is a schematic sectional view of a measurement apparatus as usedfor measuring the capillary absorption capacity (CSF) for the 0.90 mass% physiological saline solution.

FIG. 4 is a view obtained by taking a photograph of the agglomeratedwater-absorbent resin particles (B1A) as obtained from Example 1.

FIG. 5 is a diagram showing how the D50 and σζ of the water-absorbingagent (D1-1A10) were determined with a logarithmic normal probabilitypaper

FIG. 6 is a graph of which: the horizontal axis shows the CSF (g/g), andthe vertical axis shows the SFC (10⁻⁷·cm³·s·g⁻¹). This graph shows thatthe water-absorbing agents as obtained from the Examples of the presentinvention have higher liquid permeability and capillary suction forcethan the comparative water-absorbing agents as obtained from theComparative Examples.

FIG. 7 is a graph of which: the horizontal axis shows the CRC (g/g), andthe vertical axis shows the SFC (10⁻⁷·cm³·s·g⁻¹). Plotted thereon arethe CRC and SFC of the surface-crosslink-treated water-absorbent resinparticles and of the water-absorbing agents according to the presentinvention, wherein the surface-crosslink-treated water-absorbent resinparticles and the water-absorbing agents according to the presentinvention are those which are described in Examples 1 to 5. It is shownthat the effects of the liquid-permeability-enhancing agent (β) areremarkably high when the CRC is less than 29 g/g.

EXPLANATION OF THE SYMBOLS

-   100: Plastic supporting cylinder-   101: Stainless metal gauze of 400 meshes-   102: Swollen gel-   103: Piston-   104: Load (weight)-   105: Petri dish-   106: Glass filter plate-   107: Filter paper-   108: 0.90 mass % physiological saline solution-   31: Tank-   32: Glass tube-   33: 0.69 mass % physiological saline solution-   34: L-tube having cock-   35: Cock-   40: Receptacle-   41: Cell-   42: Stainless metal gauze-   43: Stainless metal gauze-   44: Swollen gel-   45: Glass filter-   46: Piston-   47: Holes in piston-   48: Collecting receptacle-   49: Balance-   1: Porous glass plate-   2: Glass filter-   3: Conduit-   4: Liquid storage container-   5: Supporting ring-   6: 0.90 mass % physiological saline solution-   7: Balance-   8: Stand-   9: Specimen to be measured (water-absorbent resin particles or    absorbing agent)-   10: Load (0.41 kPa (0.06 psi))-   11: Air-intake pipe

DETAILED DESCRIPTION OF THE INVENTION

First of all, the abbreviations as hereinafter used are defined.

The CRC refers to the absorption capacity without load.

The SFC refers to the saline flow conductivity for a 0.69 mass %physiological saline solution.

The CSF refers to the capillary absorption capacity for a 0.90 mass %physiological saline solution.

The AAP refers to the absorption capacity under load.

The D50 refers to the mass-average particle diameter.

The σζ refers to the logarithmic standard deviation of the particlediameter distribution.

The physiological saline solution refers to an aqueous sodium chloridesolution.

Hereinafter, the present invention is described in detail. Incidentally,hereinafter, the water-absorbing agent (favorably, a water-absorbentresin composition comprising the water-absorbent resin particles (α) andthe liquid-permeability-enhancing agent (β)) in the present inventionrefers to a material which comprises a water-absorbent resin having acrosslinked structure (hereinafter referred to simply as water-absorbentresin) as a main component (favorably in an amount of 50 to 100 mass %(or weight %: in the present invention, the weight and the mass have thesame meaning, and their uses herein are unified into the mass), morefavorably 80 to 100 mass %, still more favorably 90 to 100 mass %),wherein the water-absorbent resin is further modified (favorablysurface-modified, particularly surface-crosslink-treated) with acrosslinking agent, and wherein the water-absorbing agent is modified byfurther comprising another component.

Hereinafter, in the present invention, the acid-group-containingwater-absorbent resin particles are referred to as water-absorbent resinparticles (a). Of such water-absorbent resin particles (a), those ofwhich the particle diameters are controlled in the limited range, forexample, those which have the mass-average particle diameter in therange of 234 to 394 μm and the σζ in the range of 0.25 to 0.45, arereferred to as water-absorbent resin particles (a1). In addition,water-absorbent resin particles which arefurther-surface-crosslink-treated irregular-shaped pulverized particlesof a crosslinked polymer of a monomer including acrylic acid and/or itssalt are referred to as water-absorbent resin particles (α).

(1) Process for Production of Water-Absorbent Resin Particles (a1):

The water-absorbent resin, usable in the present invention, refers to ahitherto known water-absorbent resin, for example, a hitherto publiclyknown crosslinked polymer which absorbs water in a large amount ofessentially not smaller than 5 times, favorably in the range of 50 to1,000 times, of the own weight in ion-exchanged water to thus form ananionic, nonionic, or cationic water-insoluble hydrogel.

This is generally a particulate water-absorbing agent of which the maincomponent is a water-absorbent resin having a crosslinked structureobtained by a process including the step of polymerizing an unsaturatedmonomer component (favorably, an acid-group-containing (particularly,carboxyl-group-containing) unsaturated monomer), wherein thewater-absorbent resin is obtained by a process including the steps ofcarrying out the above polymerization in a state of a monomer solution(favorably, an aqueous monomer solution), and then, if necessary, dryingthe resultant polymer, and then usually pulverizing the polymer beforeand/or after the drying step. Examples of such a water-absorbent resininclude one or two or more of such as: partially-neutralized polymers ofpoly(acrylic acids); hydrolyzed graft polymers of starch-acrylonitrile;graft polymers of starch-acrylic acid; saponified copolymers of vinylacetate-acrylic acid ester; hydrolyzed copolymers of acrylonitrile oracrylamide, or crosslinked polymers of these hydrolyzed copolymers;modified polymers of carboxyl-group-containing crosslinked polyvinylalcohols; and crosslinked copolymers of isobutylene-maleic anhydride.

As to the water-absorbent resin, one kind of water-absorbent resin or amixture of water-absorbent resins is used. Above all, anacid-group-containing water-absorbent resin is favorable, and one kindof carboxyl-group-containing water-absorbent resin (which is acarboxylic acid or its salt)) or a mixture of such resins is morefavorable. Typically, there is favorably used a polymer which isobtained by a process including the step of crosslink-polymerizing amonomer including acrylic acid and/or its salt (neutralized material) asthe main component, that is, a crosslinked poly(acrylic acid) (salt)polymer which contains a grafted component if necessary.

In addition, the above water-absorbent resin needs to be water-swellableand water-insoluble. The water-extractable component (water-solublepolymer) content of the water-absorbent resin as used is favorably nothigher than 50 mass %, more favorably not higher than 25 mass %/o, stillmore favorably not higher than 20 mass %, yet still more favorably nothigher than 15 mass %, particularly favorably not higher than 10 mass %.

As examples of the above acrylic acid salt, there can be cited such as:alkaline metal (e.g. sodium, potassium, lithium) salts, ammonium salts,and amine salts of acrylic acid. The above water-absorbent resinfavorably contains, as its constitutional units, acrylic acid in therange of 0 to 50 mol % and an acrylic acid salt in the range of 100 to50 mol % (wherein the total of both is not more than 100 mol %), morefavorably, acrylic acid in the range of 10 to 40 mol % and an acrylicacid salt in the range of 90 to 60 mol % (wherein the total of both isnot more than 100 mol %). Incidentally, the molar ratio between theseacid and salt is referred to as neutralization degree. Theneutralization of the water-absorbent resin for forming the above saltmay be carried out in a monomer state before the polymerization, or maybe carried out in a polymer state on the way of or after thepolymerization, or may be carried out both in these states.

The monomer to obtain the water-absorbent resin as used in the presentinvention may further include monomers other than the above acrylic acid(salt) when the occasion demands. There is no especial limitation on themonomers other than the acrylic acid (salt). However, specific examplesthereof include: anionic unsaturated monomers (e.g. methacrylic acid,maleic acid, vinylsulfonic acid, styrenesulfonic acid,2-(meth)acrylamido-2-methylpropanesulfonic acid,2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonicacid) and their salts; nonionic-hydrophilic-group-containing unsaturatedmonomers (e.g. acrylamide, methacrylamide, N-ethyl(meth)acrylamide,N-n-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate,2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol(meth)acrylate, polyethylene glycol mono(meth)acrylate, vinylpyridine,N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine,N-vinylacetamide); and cationic unsaturated monomers (e.g.N,N-dimethylaminoethyl(meth)acrylate,N,N-diethylaminoethyl(meth)acrylate,N,N-dimethylaminopropyl(meth)acrylate,N,N-dimethylaminopropyl(meth)acrylamide, and their quaternary salts).These monomers may be used either alone respectively or in appropriatecombinations with each other.

In the present invention, when the monomers other than the acrylic acid(salt) are used, the ratio of these monomers other than the acrylic acid(salt) is favorably not more than 30 mol %, more favorably not more than10 mol %, relative to the total of the acrylic acid and/or its salt usedas the main component. If the above monomers other than the acrylic acid(salt) are used in the above ratio, then the absorption properties ofthe water-absorbent resin (water-absorbing agent) as finally obtainedare still more enhanced, and further, the water-absorbent resin(water-absorbing agent) can be obtained at still lower costs.

When the above monomer is polymerized in order to obtain thewater-absorbent resin as used in the present invention, it is possibleto carry out bulk polymerization or precipitation polymerization.However, from the viewpoints of the performance, the facility ofpolymerization control, and further the absorption properties of aswollen gel, it is favorable to carry out aqueous solutionpolymerization or reversed-phase suspension polymerization in which theabove monomer is used in the form of an aqueous solution. Suchpolymerization methods have hitherto been known in public and aredisclosed in such as U.S. Pat. No. 4,625,001, U.S. Pat. No. 4,769,427,U.S. Pat. No. 4,873,299, U.S. Pat. No. 4,093,776, U.S. Pat. No.4,367,323, U.S. Pat. No. 4,446,261, U.S. Pat. No. 4,683,274, U.S. Pat.No. 4,690,996, U.S. Pat. No. 4,721,647, U.S. Pat. No. 4,738,867, U.S.Pat. No. 4,748,076, and EP 1178059. Incidentally, in the case where theabove monomer is used in the form of an aqueous solution, theconcentration of the monomer in this aqueous solution (hereinafterreferred to as aqueous monomer solution) depends on the temperature ofthe aqueous solution or the kind of the monomer and is therefore notespecially limited. However, this concentration is favorably in therange of 10 to 70 mass %, more favorably 20 to 60 mass %. In addition,when the above aqueous solution polymerization is carried out, a solventother than water may be used jointly therewith if necessary. The kind ofthis solvent which is jointly used is not especially limited.

Examples of the method for the aqueous solution polymerization include:a method in which the aqueous monomer solution is polymerized while theresulting gel is crushed in a twin-arm type kneader; and a method inwhich the aqueous monomer solution is supplied into a predeterminedcontainer or onto a moving belt to carry out the polymerization and thenthe resultant gel is pulverized with such as a meat chopper.

When the above polymerization is initiated, there can be used, forexample, the following: radical polymerization initiators such aspotassium persulfate, ammonium persulfate, sodium persulfate, t-butylhydroperoxide, hydrogen peroxide, and2,2′-azobis(2-amidinopropane)dihydrochloride; and photoinitiators suchas 2-hydroxy-2-methyl-1-phenyl-propan-1-one.

Furthermore, a redox initiator is also available by using the abovepolymerization initiator jointly with a reducing agent which promotesthe decomposition of the above polymerization initiator and thuscombining both with each other. Examples of the above reducing agentinclude: (bi)sulfurous acid (salts) such as sodium sulfite and sodiumhydrogensulfite; L-ascorbic acid (salts); reducible metals (salts) suchas ferrous salts; and amines. However, there is no especial limitationthereto.

The amount of the above polymerization initiator as used is favorably inthe range of 0.001 to 2 mol %, more favorably 0.01 to 0.1 mol %. In thecase where the amount of the above polymerization initiator as used issmaller than 0.001 mol %, there are disadvantages in that: the amount ofunreacted monomers increases, and therefore the amount of residualmonomers increases in the resultant water-absorbent resin orwater-absorbing agent. On the other hand, in the case where the amountof the above polymerization initiator as used is larger than 2 mol %,there may be disadvantages in that the water-extractable componentcontent in the resultant water-absorbent resin or water-absorbing agentincreases.

In addition, the initiation of the polymerization reaction may becarried out by irradiating the reaction system with active energy rayssuch as radiations, electron rays, and ultraviolet rays. Furthermore,the above polymerization initiator may be used jointly therewith.Incidentally, the reaction temperature in the above polymerizationreaction is not especially limited. However, the reaction temperature isfavorably in the range of 10 to 130° C., more favorably 15 to 120° C.,particularly favorably 20 to 100° C. In addition, the reaction durationor polymerization pressure is also not especially limited, but may beset appropriately for such as the kind of the monomer or polymerizationinitiator and the reaction temperature.

The aforementioned water-absorbent resin may be a self-crosslinked-typewater-absorbent resin obtained without any crosslinking agent, but it ispreferably a water-absorbent resin obtained by copolymerization orreaction with a crosslinking agent having at least two polymerizableunsaturated groups and/or at least two reactive groups per molecule(internal-crosslinking agent for water-absorbent resins) or with acrosslinking agent which is a cyclic compound and will have at least tworeactive groups per molecule by its ring-opening reaction.

Specific examples of these internal-crosslinking agents include:N,N′-methylenebis(meth)acrylamide, (poly)ethylene glycoldi(meth)acrylate, (poly)propylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, glycerol tri(meth)acrylate,glycerol acrylate methacrylate, ethylene-oxide-modifiedtrimethylolpropane tri(meth)acrylate, pentaerythritolhexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallylphosphate, triallylamine, poly(meth)allyloxyalkanes, (poly)ethyleneglycol diglycidyl ether, glycerol diglycidyl ether; polyhydric alcoholssuch as ethylene glycol, polyethylene glycol, propylene glycol,1,3-butanediol, 1,4-butanediol, glycerol, and pentaerythritol; andethylenediamine, ethylene carbonate, propylene carbonate,polyethyleneimine, and glycidyl(meth)acrylate.

These internal-crosslinking agents may be used either alone respectivelyor in appropriate combinations with each other. In addition, theseinternal-crosslinking agents may be added to the reaction system eitherin a lump or divisionally. In the case where at least one or two or morekinds of internal-crosslinking agents are used, it is favorable, inconsideration of such as absorption properties of the finally obtainedwater-absorbent resin or water-absorbing agent, that a compound havingat least two polymerizable unsaturated groups is essentially used duringthe polymerization.

The amount of the above internal-crosslinking agent as used is favorablyin the range of 0.001 to 2 mol %, more favorably 0.02 to 1.0 mol %,still more favorably 0.06 to 0.30 mol %, particularly favorably 0.03 to0.15 mol %, relative to the aforementioned monomer (exclusive of theinternal-crosslinking agents). In the case where the amount of the aboveinternal-crosslinking agent as used is smaller than 0.001 mol % orlarger than 2 mol %, there is a possibility that no sufficientabsorption properties can be obtained.

In the case where the above internal-crosslinking agent is used tointroduce a crosslinked structure into the inside of the polymer, it isenough that the above internal-crosslinking agent is added to thereaction system before, on the way of, or after the polymerization ofthe above monomer, or after its neutralization. However, it is favorableto carry out the addition before the polymerization.

Incidentally, when the above polymerization is carried out, to thereaction system there can be added such as: hydrophilic polymers (e.g.starch, cellulose, starch derivatives, cellulose derivatives, polyvinylalcohol, poly(acrylic acid) (salts), and crosslinked poly(acrylic acid)(salts)) in an amount of 0 to 50 mass % (relative to the monomer); andothers (e.g. various foaming agents such as (hydrogen)carbonates, carbondioxide, azo compounds, and inert organic solvents; various surfactants;chelating agents; chain transfer agents such as hypophosphorous acid(salts); inorganic fine particles such as kaolin, talc, and silicondioxide; polyvalent metal salts such as poly(aluminum chloride),aluminum sulfate, and magnesium sulfate) in an amount of 0 to 10 mass %(relative to the monomer).

When the above crosslinked polymer is a gel as obtained by the aqueoussolution polymerization, namely, a crosslinked hydrogel polymer, thenthe crosslinked polymer is dried, if necessary, and usually pulverizedbefore and/or after this drying, thus forming the water-absorbent resinparticles (a). In addition, the drying is carried out in the temperaturerange of usually 60 to 250° C., favorably 100 to 220° C., more favorably120 to 200° C. The drying duration depends on the surface area and watercontent of the polymer and the kind of the dryer and is selected for thewater content to be an objective one.

The water content of the water-absorbent resin usable in the presentinvention (defined as the amount of water contained in thewater-absorbent resin and measured by the drying loss at 180° C. in 3hours) is not especially limited. However, the water content isfavorably such that the water-absorbent resin can be flowable even atroom temperature, such as in the form of particles, a powder, or aparticulate dried material agglomerate, and is more favorably such thatthe water-absorbent resin can be in a powder state having a watercontent of 0.2 to 30 mass %, still more favorably 0.3 to 15 mass %,particularly favorably 0.5 to 10 mass %. In the case where the watercontent is high, there is a possibility not only that the flowabilitymay be so poor as to hinder the production, but also that thewater-absorbent resin cannot be pulverized or controlled to the specificparticle diameter distribution.

In addition, examples of the water-absorbent resin usable in the presentinvention include those which are of the irregular shape and easy topulverize, such as in the form of particles, a powder, or a particulatedried material agglomerate.

The water-absorbent resin in the form of particles, a powder, or aparticulate dried material agglomerate, which is obtained by theaforementioned process, is pulverized with a pulverizer. Thewater-absorbent resin particles (a) or (a1) can be obtained by thepulverization. Although not especially limited, examples of usablepulverizers include roll type pulverizers (e.g. roll mills), hammer typepulverizers (e.g. hammer mills), impact type pulverizers, cutter mills,turbo grinders, ball mills, and flash mills. Of these, the roll millsare favorable for controlling the particle diameter distribution. Thepulverization may be carried out continuously twice or more forcontrolling the particle diameter distribution. However, the number oftimes of the pulverization is favorable 3 or more. In the case where thepulverization is carried out twice or more, the pulverizer as used eachtime may be either the same or different. It is also possible to usedifferent ends of pulverizers in combination.

The pulverized water-absorbent resin particles (a) may be classifiedwith a sieve of a specific mesh opening size in order to control theresin particles (a) to the specific particle diameter distribution.Although not especially limited, examples of classifiers as used includeshaking sieves (e.g. unbalanced-weight driving types, resonance types,shaking motor types, electromagnetic types, circular shaking types),in-plane motion sieves (e.g. horizontal motion types, horizontalcircle-straight line motion types, three-dimensional circular motiontypes), movable-mesh type sieves, forced-stirring type sieves,mesh-face-shaking type sieves, wind power sieves, and sound wave sieves.Favorably, the shaking sieves and the in-plane motion sieves are used.The sieve mesh opening size favorable for obtaining the water-absorbentresin particles (a1) usable in the present invention is in the range of1,000 to 300 μm, more favorably 900 to 400 μm, most favorably 710 to 450μm. Out of these ranges, there is a possibility that the objectiveparticle diameter distribution cannot be obtained.

For the purpose of controlling the water-absorbent resin particles (a)usable in the present invention to the specific particle diameterdistribution, the water-absorbent resin particles (a) usable in thepresent invention may be further classified to thereby remove a portionor all of particles smaller than a specific particle diameter. Althoughnot especially limited, examples of classifiers as favorably used inthis step include the aforementioned exemplifying ones. Besides them,such as fine-powder type classification apparatuses (e.g. centrifugalforce types, inertial force types) are used. In this step, a portion orall of particles having particle diameters favorably smaller than 200μm, more favorably smaller than 150 μm, most favorably smaller than 106μm, are removed in order to obtain the water-absorbent resin particles(a1) usable in the present invention.

In addition, in the present invention, favorably, there may be involveda agglomeration step in which the particles as removed by theaforementioned classification are regenerated as larger particles or aparticulate agglomerate by such as agglomeration to thus enable their orits use as the water-absorbent resin particles (a1) usable in thepresent invention. In this agglomeration step, publicly known arts toregenerate a fine powder are usable. Examples of such usable artsinclude methods in which: warm water and a fine powder of awater-absorbent resin are mixed together and then dried (U.S. Pat. No.6,228,930); a fine powder of a water-absorbent resin is mixed with anaqueous monomer solution, and then the resultant mixture is polymerized(U.S. Pat. No. 5,264,495); water is added to a fine powder of awater-absorbent resin, and then the resultant mixture is agglomeratedunder not less than a specific face pressure (EP 0844270); a fine powderof a water-absorbent resin is sufficiently wetted to form an amorphousgel, and then this gel is dried and pulverized (U.S. Pat. No.4,950,692); and a fine powder of a water-absorbent resin and a polymergel are mixed together (U.S. Pat. No. 5,478,879). However, there isfavorably used the aforementioned method in which warm water and a finepowder of a water-absorbent resin are mixed together and then dried. Inaddition, the water-absorbent resin obtained from the agglomeration stepmay be either used, as it is, as the water-absorbent resin particles(a1) usable in the present invention, or returned to the aforementionedpulverization step and/or classification step. However, for obtainingthe objective water-absorbent resin particles (a1), the return to thepulverization step and/or classification step is favorable. Thewater-absorbent resin particles (a), as regenerated in this way,substantially have a porous structure. The ratio of the water-absorbentresin, as regenerated in the agglomeration step and contained in thewater-absorbent resin particles (a1) usable in the present invention, isfavorably not less than 10 mass %, more favorably not less than 15 mass%, most favorably not less than 20 mass %. In the case where used as thewater-absorbent resin particles (a1) usable in the present invention,the water-absorbent resin as regenerated in the agglomeration step has alarger surface area and therefore gives a larger capillary suction forcethan a unregenerated water-absorbent resin and is thus advantageous overit in performance.

(2) Features of Water-Absorbent Resin Particles (a1):

The water-absorbent resin particles (a1) usable in the present inventionhave the following features.

Examples of the shape of the water-absorbent resin particles (a1) usablein the present invention include spherical shape, fibrous shape, barshape, approximately spherical shape, flat shape, irregular shape,agglomerated particle shape, and porous particle shape without beingespecially limited. However, the irregularly pulverized shape asobtained via the pulverization step is favorably usable. In addition,favorably, the water-absorbent resin particles (a1) usable in thepresent invention partially contain the particles having a porousstructure (which may be a foamed structure) and the water-absorbentresin particles (a) as regenerated in the aforementioned agglomerationstep, and their ratio is favorably not less than 10 mass %, morefavorably not less than 15 mass %, most favorably not less than 20 mass%. Furthermore, the bulk density (defined by JIS K-3362) of thewater-absorbent resin particles (a1) is favorably in the range of 0.40to 0.90 g/ml, more favorably 0.50 to 0.80 g/ml, from the viewpoint ofexcellent properties of the water-absorbing agent.

As to the particle diameters of the water-absorbent resin particles (a1)usable in the present invention, there are favorably used those whichhave a mass-average particle diameter in the range of favorably 10 to1,000 μm, more favorably 100 to 800 μm, still more favorably 200 to 400μm, particularly favorably 250 to 380 μm.

As to the water-absorbent resin particles (a1) usable in the presentinvention, the mass ratio (particles having particle diameters of notsmaller than 300 μm)/(particles having particle diameters of smallerthan 300 μm) is favorably in the range of 80/20 to 20/80, more favorably78/22 to 30/70, particularly favorably 75/25 to 40/60.

Favorable ranges of the particle diameter distribution are hereinaftershown further in addition to the above.

The mass ratio (particles having particle diameters of smaller than 300μm but not smaller than 150 μm)/(particles having particle diameters ofsmaller than 150 μm) is favorably in the range of 100/0 to 50/50, morefavorably 99.5/0.5 to 65/35, particularly favorably 99/1 to 75/25.

The mass ratio (particles having particle diameters of not smaller than500 μm)/(particles having particle diameters of smaller than 500 μm butnot smaller than 300 μm) is favorably in the range of 60/40 to 0/100,more favorably 50/50 to 0/100, particularly favorably 40/60 to 0/100.

The water-absorbent resin particles (a1) usable in the present inventionare favorably regulated to the aforementioned particle diameterdistribution, whereby there can be obtained the water-absorbing agentaccording to the present invention which is excellent in both of theliquid permeability and the capillary suction force.

Incidentally, the “particles having particle diameters of not smallerthan 300 μm”, as referred to in the present invention, refers toparticles remaining on a mesh of the mesh opening size of 300 μm asmeasured after having been classified by the below-mentionedclassification method. In addition, the “particles having particlediameters of smaller than 300 μm” similarly refers to particles havingpassed through the mesh of the mesh opening size of 300 μm as measuredafter having been classified by the below-mentioned classificationmethod. The same reference is applied also to the other mesh openingsizes (e.g. 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 212 μm, 200 μm, 150μm, 45 μm). Incidentally, for example, in the case where 50 mass % ofparticles are classified with the mesh of the mesh opening size of 300μm, the mass-average particle diameter (D50) is 300 μm.

The water-absorbent resin particles (a1), as obtained by the aboveprocess, display an absorption capacity without load in the range offavorably 15 to 50 g/g, more favorably 20 to 40 g/g, most favorably 25to 35 g/g, for a 0.9 mass % physiological saline solution without load.The properties such as this absorption capacity without load areadjusted appropriately for the purpose. However, in the case where thisabsorption capacity without load is less than 15 g/g or more than 50g/g, there is a possibility that the water-absorbing agent according tothe present invention cannot be obtained.

The water-absorbent resin particles (a1), as obtained by the aboveprocess, have a crosslinked structure. The water-extractable componentcontent of the water-absorbent resin particles as used is favorably nothigher than 25 mass %, more favorably not higher than 20 mass %, stillmore favorably not higher than 15 mass %, particularly favorably nothigher than 10 mass %. The water-extractable component content of thewater-absorbent resin particles is measured by the below-mentionedmethod.

(3) Process for Production of Water-Absorbing Agent:

The water-absorbing agent used in the present invention is favorablyobtained by a process including the steps of: crosslink-treatingsurfaces of the water-absorbent resin particles (a1), as obtained by theaforementioned process, with a specific surface-crosslinking agent; andadding the liquid-permeability-enhancing agent (β).

Favorably, the surface-crosslinking is carried out to such a degree thatthe absorption capacity without load (CRC) will be in the range of 15 to33 g/g (but not including 33 g/g) and that the absorption capacity underload (AAP) will be in the range of 15 to 29 g/g.

As examples of surface-crosslinking agents as favorably used in thepresent invention, there can be cited compounds which have at least twofunctional groups reactable with a functional group of thewater-absorbent resin (wherein the at least two functional groups are,favorably, functional groups which can make a dehydration reaction ortransesterification reaction with a carboxyl group). The functionalgroup of the water-absorbent resin is favorably an anionic dissociatinggroup and more favorably the carboxyl group.

Examples of such a surface-crosslinking agent include:

polyhydric alcohol compounds (e.g. ethylene glycol, diethylene glycol,propylene glycol, triethylene glycol, tetraethylene glycol, polyethyleneglycol, 1,3-propanediol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol,polyglycerol, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol,1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine,polyoxypropylene, oxyethylene-oxypropylene block copolymers,pentaerythritol, and sorbitol);epoxy compounds (e.g. ethylene glycol diglycidyl ether, polyethyleneglycol diglycidyl ether, glycerol polyglycidyl ether, diglycerolpolyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycoldiglycidyl ether, polypropylene glycol diglycidyl ether, and glycidol);polyamine compounds (e.g. ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentaamine, pentaethylenehexamine,and polyethyleneimine) and their inorganic or organic salts (e.g.azetidinium salts);polyisocyanate compounds (e.g. 2,4-tolylene diisocyanate andhexamethylene diisocyanate);aziridine compounds (e.g. polyaziridine);polyoxazoline compounds (e.g. 1,2-ethylenebisoxazoline, bisoxazoline,and polyoxazoline);carbonic acid derivatives (e.g. urea, thiourea, guanidine,dicyandiamide, 2-oxazolidinone);alkylene carbonate compounds (e.g. 1,3-dioxolan-2-one,4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one,4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one,4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one,4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and1,3-dioxopan-2-one);haloepoxy compounds (e.g. epichlorohydrin, epibromohydrin, andα-methylepichlorohydrin) and their polyamine addition products (e.g.Kymene (registered trademark) produced by Hercules);oxetane compounds;silane coupling agents (e.g. γ-glycidoxypropyltrimethoxysilane andγ-aminopropyltriethoxysilane); andpolyvalent metallic compounds (e.g. hydroxides or chlorides or sulfatesor nitrates or carbonates of such as zinc, calcium, magnesium, aluminum,iron and zirconium). These may be used either alone respectively or incombinations with each other.

In addition, the amount of the above surface-crosslinking agent as usedis favorably in the range of 0.001 to 10 mass parts, more favorably 0.01to 5 mass parts, per 100 mass parts of the water-absorbent resinparticles (a1). In the case where this amount is larger than 10 massparts, not only are there economical disadvantages in that noperformance corresponding thereto is obtained, but also thesurface-crosslinking agent remains unfavorably in a large amount.Furthermore, in the case where this amount is smaller than 0.001 masspart, there is a possibility that the resultant saline flow conductivity(SFC) for a 0.69 mass % physiological saline solution may beinsufficient.

In addition, such as inorganic acids and organic acids may be used inorder to more accelerate the reaction of the surface-crosslinking agentto thus more enhance the absorption properties. Examples of theseinorganic acids and organic acids include sulfuric acid, phosphoricacid, hydrochloric acid, citric acid, glyoxylic acid, glycolic acid,glycerol phosphate, glutaric acid, cinnamic acid, succinic acid, aceticacid, tartaric acid, lactic acid, pyruvic acid, fumaric acid, propionicacid, 3-hydroxypropionic acid, malonic acid, butyric acid, isobutyricacid, imidinoacetic acid, malic acid, isethionic acid, citraconic acid,adipic acid, itaconic acid, crotonic acid, oxalic acid, salicylic acid,gallic acid, sorbic acid, gluconic acid, and p-toluenesulfonic acid. Inaddition, there may be used those which are disclosed in EP 0668080,such as inorganic acids, organic acids, and polyamino acids. The amountof these materials as used differs according to such as pH of thewater-absorbent resin, but is favorably in the range of 0 to 10 massparts, more favorably 0.1 to 5 mass parts, per 100 mass parts of thewater-absorbent resin particles (a1).

In the present invention, when the water-absorbent resin particles (a1)and the surface-crosslinking agent are mixed together, water isfavorably used as a solvent. The amount of water, as used, depends uponsuch as type or particle diameters of the water-absorbent resinparticles (a1), but is favorably larger than 0 mass part but not largerthan 20 mass parts, more favorably in the range of 0.5 to 10 mass parts,still more favorably 0.5 to 5 mass parts, per 100 mass parts of thesolid content of the water-absorbent resin particles (a1).

In addition, when the water-absorbent resin particles (a1) and thesurface-crosslinking agent are mixed together, a hydrophilic organicsolvent may be used as a solvent, if necessary. Examples of thehydrophilic organic solvent include: lower alcohols (e.g. methylalcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butylalcohol, isobutyl alcohol, and t-butyl alcohol); ketones (e.g. acetone);ethers (e.g. dioxane, tetrahydrofuran, and alkoxypolyethylene glycol);amides (e.g. N,N-dimethylformamide); and sulfoxides (e.g. dimethylsulfoxide). The amount of the hydrophilic organic solvent, as used,depends upon such as type or particle diameters of the water-absorbentresin particles (a1), but is favorably not larger than 20 mass parts,more favorably not larger than 10 mass parts, still more favorably notlarger than 5 mass parts, per 100 mass parts of the solid content of thewater-absorbent resin particles (a1).

In addition, when the water-absorbent resin particles (a1) and thesurface-crosslinking agent are mixed together, there may be caused tocoexist a noncrosslinkable water-soluble inorganic base (favorably:alkaline metal salts, ammonium salts, alkaline metal hydroxides,water-soluble aluminum salts, ammonia or its hydroxide) and anirreducible alkaline-metal-salt pH buffer (favorably such ashydrogencarbonates, dihydrogenphosphates, and hydrogenphosphates) forthe purpose of more uniformly mixing the water-absorbent resin particles(a1) and the surface-crosslinking agent together. The amount of thesematerials, as used, depends upon such as type or particle diameters ofthe water-absorbent resin particles (a1), but is favorably in the rangeof 0.005 to 10 mass parts, more favorably 0.05 to 5 mass parts, per 100mass parts of the solid content of the water-absorbent resin particles(a1).

In addition, when the water-absorbent resin particles (a1) are mixedwith the surface-crosslinking agent, for example, there may be used amethod in which: the water-absorbent resin particles (a1) are dispersedinto the above hydrophilic organic solvent, and then thesurface-crosslinking agent is added to the resultant dispersion.However, in a favorable method, the surface-crosslinking agent, which isdissolved or dispersed in water and the hydrophilic organic solvent ifnecessary, is spraywise or dropwise added directly to thewater-absorbent resin particles (a1) under stirring. In addition, whenthe mixing is carried out with water, there may be made to coexist suchas a water-insoluble inorganic fine particle powder, a water-solublepolyvalent metal, or a surfactant.

A mixing apparatus, as used when the water-absorbent resin particles(a1) and the surface-crosslinking agent are mixed together, has greatmixing power favorably for uniformly and surely mixing both. Favorableexamples of the above mixing apparatus include cylinder type mixers,double-wall cone type mixers, V-character-shaped mixers, ribbon typemixers, screw type mixers, fluidized-furnace rotary disk type mixers,gas current type mixers, twin-arm kneaders, internal mixers, pulverizingtype kneaders, rotary mixers, and screw type extruders.

After the water-absorbent resin particles (a1) and thesurface-crosslinking agent have been mixed together, the resultantmixture is subjected to the heat treatment and light irradiationtreatment, whereby surfaces of the water-absorbent resin particles (a1)are crosslinked. Favorably, this surface-crosslinking is carried out tosuch a degree that the absorption capacity without load (CRC) will be inthe range of 15 to 33 g/g (but not including 33 g/g) and that theabsorption capacity under load (AAP) will be in the range of 15 to 29g/g. When the heat treatment is carried out in the present invention,the treating time is favorably in the range of 1 to 180 minutes, morefavorably 3 to 120 minutes, still more favorably 5 to 100 minutes. Thetreating temperature is favorably in the range of 60 to 250° C., morefavorably 100 to 210° C., still more favorably 120 to 200° C. In thecase where the heating temperature is lower than 60° C., there is apossibility not only that the heat treatment may take so much time as tocause the lowering of the productivity, but also that no uniformcrosslinking may be achieved and therefore no objective water-absorbingagent can be obtained. In addition, in the case where the heatingtemperature is higher than 250° C., the resultantsurface-crosslink-treated water-absorbent resin particles (α) aredamaged and therefore there are cases where it is difficult to obtainthe water-absorbing agent which is excellent in the absorption capacity.

The aforementioned heat treatment can be carried out with conventionaldryers or heating furnaces. Examples of the above dryers include channeltype mixing dryers, rotary dryers, disk dryers, fluidized-bed dryers,air blow type dryers, and infrared dryers. In the case where the lightirradiation treatment is carried out in place of the heat treatment inthe present invention, it is favorable to irradiate ultraviolet rays,and besides, photoinitiators are usable.

In the case where the water-absorbent resin particles (a1) have beenheated in the aforementioned surface treatment step, it is favorable tocool the heated water-absorbent resin particles. It is favorable thatthe cooling is carried out until the temperature falls into the range of100 to 20° C. In addition, examples of coolers as used for the coolinginclude apparatuses in which the heating media of the above dryers asused for the heat treatment are replaced with cooling media.

As to the water-absorbing agent as obtained via the aforementionedsteps, its particle diameter distribution is favorably regulated by theparticle regulation step.

If necessary, the above process for production of the water-absorbingagent according to the present invention may further comprise a step forproviding the water-absorbing agent or water-absorbent resin particleswith various functions, for example, a step of adding such as:deodorants; antibacterial agents; perfumes; foaming agents; pigments;dyes; hydrophilic short fibers; plasticizers; pressure-sensitiveadhesives; metal soap; surfactants; manure; oxidants; reducing agents;water; salts; chelating agents; fungicides; hydrophilic polymers (e.g.polyethylene glycol); paraffins; hydrophobic polymers; thermoplasticresins (e.g. polyethylene, polypropylene); and thermosetting resins(e.g. polyester resins, urea resins). The amount of these additives asused is favorably in the range of 0 to 10 mass parts, more favorably 0to 1 mass part, per 100 mass parts of the water-absorbing agent.

The liquid-permeability-enhancing agent (β), as referred to in thepresent invention, refers to a substance such that the SFC of thewater-absorbing agent as obtained by mixing thesurface-crosslink-treated water-absorbent resin particles (α) and theliquid-permeability-enhancing agent (β) together can be higher than theSFC of the water-absorbent resin particles (α) to which theliquid-permeability-enhancing agent (β) is not added. The addition ofthe liquid-permeability-enhancing agent (β) may be carried out any timeof before, during, and after the surface treatment. Theliquid-permeability-enhancing agent (β) has an effect of enhancing theliquid permeability by spreading the spaces between swollenwater-absorbent resin particles due to such as a role like a spacer oran ionic surface-crosslinking effect. On the other hand, theliquid-permeability-enhancing agent (β) further has an effect ofdeteriorating the capillary suction force. However, surprisingly, thewater-absorbing agent according to the present invention, which has beencontrolled to the specific range of particle diameter distribution, isexcellent in the liquid permeability and the capillary suction force,and therefore can maintain high capillary suction force though itcontains the liquid-permeability-enhancing agent (β). In addition,surprisingly, in the case where the water-absorbing agent according tothe present invention, which has been controlled to the specific rangeof particle diameter distribution, contains theliquid-permeability-enhancing agent (β), itsliquid-permeability-enhancing effect is much higher than conventional.That is to say, usually, the SFC varies greatly with the particlediameter distribution. Specifically, as the average particle diameterbecomes smaller, the SFC value becomes smaller. However, the presentinventors have discovered a feature that the SFC of the water-absorbingagent which contains the liquid-permeability-enhancing agent (β) dependsonly on the CRC regardless of the particle diameter distribution of thewater-absorbing agent if this particle diameter distribution is in acertain specific range. On the other hand, the CSF of thewater-absorbing agent which contains the liquid-permeability-enhancingagent (β) depends on the particle diameter distribution. Therefore, forthe water-absorbing agent, which has been controlled to a certainspecific particle diameter distribution, to contain theliquid-permeability-enhancing agent (β) has made it possible to obtainthe water-absorbing agent which is excellent in both of the SFC and CSF.

Examples of the liquid-permeability-enhancing agent (β), as used in thepresent invention, include hydrophilic inorganic compounds, and thereare favorably used such as water-insoluble hydrophilic inorganic fineparticles and water-soluble polyvalent metal salts. As to thehydrophilicity as referred to in the present invention, for example,there can be cited those which have a hydrophilicity of not less than70% as disclosed in EP 0629411. In the present invention, such ascationic high-molecular compounds (e.g. those which are cited asexamples on column 11 of U.S. Pat. No. 5,797,893) and hydrophobicinorganic fine particles enhance the liquid permeability, but increasethe contact angle of the water-absorbing agent to bring about greatdeterioration of the CSF. Therefore, as the case may be, it isunfavorable that they are used. Such a surfactant as deteriorates thesurface tension of the water-absorbing agent brings about greatdeterioration of the CSF. Therefore, it is unfavorable that such asurfactant is used in the present invention.

In the case where the liquid-permeability-enhancing agent (β) as used inthe present invention is in the form of inorganic fine particles, theirparticle diameters are favorably not larger than 500 μm, more favorablynot larger than 100 μm, most favorably not larger than 10 μm, from theviewpoint of the handling property and the addition effects. Theaforementioned particle diameters include both a case of particlediameters of primary particles and a case of particle diameters ofsecondary particles (agglomerated materials, agglomerates). In the casewhere particles of compounds of which the particles have high hardnessand are not easily destroyed by impact, such as silica and alumina thatare non-agglomerates (primary particles), are used, the particlediameters of primary particles of the agglomerates or agglomeratedmaterials are favorably not larger than 5 μm, more favorably not largerthan 1 μm, most favorably not larger than 0.1 μm.

Specific examples of these liquid-permeability-enhancing agents (β) asused in the present invention include: mineral products such as talc,kaolin, fuller's earth, bentonite, activated clay, barite, naturalasphaltum, strontium ore, ilmenite, and pearlite; aluminum compoundssuch as aluminum sulfate tetradeca- to octadecahydrates (or anhydride),potassium aluminum sulfate dodecahydrate, sodium aluminum sulfatedodecahydrate, ammonium aluminum sulfate dodecahydrate, aluminumchloride, poly(aluminum chloride), and aluminum oxide; other polyvalentmetal salts, polyvalent metal oxides, and polyvalent metal hydroxides;hydrophilic amorphous silica (e.g. dry method: Reolosil QS-20 ofTokuyama Corporation, precipitation method: Sipernat 22S and Sipernat2200 of DEGUSSA Corporation); and oxide composites such as siliconoxide-aluminum oxide-magnesium oxide composite (Attagel #50 of ENGELHARDCorporation), silicon oxide-aluminum oxide composite, and siliconoxide-magnesium oxide composite. In addition, those which are cited asexamples in such as U.S. Pat. No. 5,164,459 and EP 0761241 are alsousable. It is favorable that the hydrophilic particles (e.g. aluminumsulfate tetradeca- to octadecahydrates and the hydrophilic amorphoussilica) are selected from among the above particles and used. However,in the case where the hydrophilicity of the particles is low, it isenough to use particles obtained by treating surfaces of particles withhydrophilic compounds to thus hydrophilize them. These may be usedeither alone respectively or in combinations with each other.

As to methods for mixing the liquid-permeability-enhancing agent (β) asused in the present invention, the mixing is carried out by such as: amethod in which the water-soluble polyvalent metal salt (e.g. aluminumsulfate) and the cationic high-molecular compound is mixed in the formof an aqueous solution, a slurry, or a powder. However, a favorablemethod is the method in which the mixing is carried out in the form of apowder. In addition, the amount of the addition is favorably in therange of 0.01 to 5 mass %, more favorably 0.05 to 3 mass %, relative tothe water-absorbent resin particles. In the case where the amount of theaddition is larger than 5 mass %, there is a possibility that theabsorption capacity may be deteriorated. In the case where the amount ofthe addition is smaller than 0.01 mass %, there is a possibility that itmay become impossible to obtain the effects of the addition. Inaddition, by changing the amount of the addition, it is possible toadjust the liquid permeability and capillary suction force of thewater-absorbing agent.

An apparatus for mixing the water-absorbent resin particles and theliquid-permeability-enhancing agent (β) together does not need to haveespecially great mixing power. For example, the mixing may be carriedout with such as disintegrators or sieving machines. Favorable examplesof the above mixing apparatus include cylinder type mixers, double-wallcone type mixers, V-character-shaped mixers, ribbon type mixers, screwtype mixers, fluidized-furnace rotary disk type mixers, gas current typemixers, twin-arm kneaders, internal mixers, pulverizing type kneaders,rotary mixers, screw type extruders, and static mixers. In addition, thetime of the addition may be any time of before the water-absorbing agentis obtained, during, and after its production in the aforementionedproduction process. However, the time of the addition is favorably afterthe surface-crosslinking.

The water-absorbing agent, as obtained in the above way, favorably hassuch as the following CRC, AAP, SFC, CSF, particle diameterdistribution, surface tension, contact angle, bulk density,water-extractable component content, shape, and water content. However,the water-absorbing agent according to the present invention may beobtained by other processes.

Besides, in the present invention, it is also possible to provide thewater-absorbing agent according to the present invention with variousfunctions by further adding such as: disinfectants; deodorants;antibacterial agents; perfumes; various inorganic powders; foamingagents; pigments; dyes; hydrophilic short fibers; manure; oxidants;reducing agents; water; and salts.

(4) Features of Water-Absorbing Agent According to Present Invention:

The water-absorbing agent according to the present invention has thefollowing features.

The water-absorbing agent according to the present invention is aparticulate water-absorbing agent comprising the water-absorbent resinparticles (α) and the liquid-permeability-enhancing agent (β), whereinthe water-absorbent resin particles (α) are obtained by a processincluding the step of crosslink-polymerizing a monomer including acrylicacid and its salt and have a crosslinked structure.

The water-absorbing agent according to the present invention includesparticles having been surface-crosslink-treated with the compound whichhas at least two functional groups reactable with a functional group ofthe water-absorbent resin (wherein the at least two functional groupsare, favorably, functional groups which can make a dehydration reactionor transesterification reaction with a carboxyl group).

The water-absorbing agent according to the present invention exhibits anabsorption capacity without load (CRC) of at least not less than 15 g/g,favorably in the range of 15 to 33 g/g (but not including 33 g/g), morefavorably 17 to 31 g/g (but not including 31 g/g), still more favorably19 to 29 g/g (but not including 29 g/g), most favorably 23 to 28 g/g(but not including 28 g/g), for a 0.9 mass % physiological salinesolution without load. In the case where the CRC is less than 15 g/g,not only is there a possibility that the absorption capacity may be toolow to obtain sufficient performance in the case of the use for such aswater-absorbent structures, but also there are economical disadvantages.In addition, in the case where the CRC is not less than 33 g/g, there apossibility that: because the gel absorption capacity may be too highand because the gel strength may accordingly be deteriorated, theenhancement of the liquid permeability (enhancement of SFC) by theliquid-permeability-enhancing agent (β) may be insufficient to obtainthe objective performances. The liquid-permeability-enhancing agent (β),as contained in the water-absorbing agent according to the presentinvention, has effects particularly when the CRC is less than 33 g/g andhas great effects when the CRC is less than 29 g/g.

The particle diameter distribution of the water-absorbing agentaccording to the present invention is favorably substantially the sameas that of the aforementioned water-absorbent resin particles (a1). Thewater-absorbing agent according to the present invention ischaracterized by combining performances excellent in both of the liquidpermeability and the capillary suction force. Needed for achieving thisis the strictly controlled particle diameter distribution. The presentinventors have discovered that the liquid permeability and the capillarysuction force change greatly at around 300 μm in particle diameter as aboundary, and thus the present inventors have utilized it for thepresent invention. That is to say, particles having particle diameterslarger than around 300 μm as a boundary display high liquidpermeability, but are inferior in the capillary suction force, whileparticles having particle diameters smaller than around 300 μm as aboundary are excellent in the capillary suction force, but their liquidpermeability is greatly deteriorated. The present inventors havecompleted the present invention by discovering that, if theabove-discovered fact is utilized to regulate the mass-average particlediameter (D50) to around 300 μm and to control the logarithmic standarddeviation (σζ) of the particle diameter distribution in the specificrange and further if the liquid-permeability-enhancing agent (β) is madeto be contained, then it becomes possible to obtain the water-absorbingagent which combines performances excellent in both of the liquidpermeability and the capillary suction force.

The particle diameter distribution of the water-absorbing agentaccording to the present invention is specifically as follows.

As to the water-absorbing agent according to the present invention, themass-average particle diameter (D50) is favorably in the range of 234 to394 μm, more favorably 256 to 363 μm, most favorably 281 to 331 μm. Asto the liquid permeability and the capillary suction force, theseperformances change greatly at around 300 μm in particle diameter as aboundary. Smaller particle diameters are advantageous to the capillarysuction force, but disadvantageous to the liquid permeability. Inaddition, larger particle diameters are advantageous to the liquidpermeability, but disadvantageous to the capillary suction force. Thatis to say, in the case where the mass-average particle diameter (D50) isnot larger than 233 μm or not smaller than 395 μm, there is apossibility that the objective water-absorbing agent according to thepresent invention, which is excellent in both of the liquid permeabilityand the capillary suction force, cannot be obtained, and that,accordingly, a water-absorbing agent which is excellent in only eitherone of them may be obtained.

As to the water-absorbing agent according to the present invention, thelogarithmic standard deviation (σζ) of the particle diameterdistribution is favorably in the range of 0.25 to 0.45, more favorably0.27 to 0.43, most favorably 0.30 to 0.40. The smaller logarithmicstandard deviation (σζ) of the particle diameter distribution shows thenarrower particle diameter distribution. However, it is important forthe water-absorbing agent according to the present invention that theparticle diameter distribution has the broadness in some degree. In thecase where the logarithmic standard deviation (σζ) of the particlediameter distribution is less than 0.25, not only is the capillarysuction force deteriorated, but also the productivity is greatlydeteriorated. In the case where the logarithmic standard deviation (σζ)of the particle diameter distribution is more than 0.45, there is apossibility that the particle diameter distribution may be too broad,thus resulting in low liquid permeability. In addition, thewater-absorbing agent according to the present invention includesparticles having particle diameters in the range of 200 μm above andbelow 300 μm (i.e. 100 to 500 μm) in an amount of favorably not smallerthan 80 mass %, more favorably not smaller than 85 mass %, relative tothe water-absorbing agent.

As to the water-absorbing agent according to the present invention, themass ratio (particles having particle diameters not smaller than 300μm)/(particles having particle diameters smaller than 300 μm) isfavorably in the range of 80/20 to 20/80, more favorably 78/22 to 30/70,particularly favorably 75/25 to 40/60.

Favorable ranges of the particle diameter distribution are hereinaftershown further in addition to the above.

The mass ratio (particles having particle diameters smaller than 300 μmbut not smaller than 150 μm)/(particles having particle diameterssmaller than 150 μm) is favorably in the range of 100/0 to 50/50, morefavorably 99.5/0.5 to 65/35, particularly favorably 99/1 to 75/25.

The mass ratio (particles having particle diameters not smaller than 500μm)/(particles having particle diameters smaller than 500 μm but notsmaller than 300 μm) is favorably in the range of 60/40 to 0/100, morefavorably 50/50 to 0/100, particularly favorably 40/60 to 0/100.

The water-absorbing agent according to the present invention isfavorably regulated to the aforementioned particle diameter distributionand thereby can have performances excellent in both of the liquidpermeability and the capillary suction force.

As the water-absorbing agent according to the present invention, thereare used those of which the water-extractable component content isfavorably not higher than 15 mass %, more favorably not higher than 13mass %, most favorably not higher than 10 mass %. In addition,particularly when the water-extractable component content of thewater-absorbing agent is not higher than 15 mass %, theliquid-permeability-enhancing agent (β) usable in the present inventionremarkably takes effect. In the case where the water-extractablecomponent content is higher than 15 mass % in the present invention,there is a possibility not only that no effects of the present inventionmay be obtained, but also that the performance may be deteriorated inthe use for water-absorbent structures. In addition, such awater-extractable component content is unfavorable also from theviewpoint of safety. As a cause of the performance deterioration, it canbe cited that, when the water-absorbing agent absorbs water to swell, ahigh-molecular component elutes from the inside of the water-absorbingagent to thereby hinder the liquid permeation. The high-molecularcomponent can be considered to resist when a liquid flows acrosssurfaces of water-absorbing agent particles. In addition, similarly, theelution of the high-molecular component has a possibility of increasingthe viscosity of an absorbed solution to thus deteriorate the capillarysuction force. The water-extractable component content of thewater-absorbing agent is measured by the below-mentioned method.

As to the water-absorbing agent according to the present invention, itis favorable that a portion of the water-absorbent resin particles (α)included in the water-absorbing agent have a porous structure (which maybe a foamed structure). The phrase “have a porous structure”, ashereupon referred to, refers to a state such as where fine particles ofthe water-absorbent resin particles (α) are agglomerated or bubbles arecontained in an amount of not smaller than 10% of the volume. Inaddition, it is more favorable that this porous structure is a structureobtained by the aforementioned agglomeration step. Above all, it is themost favorable that the porous structure is that of afine-particles-agglomerated material obtained by a process as disclosedin U.S. Pat. No. 6,228,930. The ratio of the water-absorbent resinparticles (α) having the porous structure is favorably not less than 10mass %, more favorably not less than 15 mass %, most favorably not lessthan 20 mass %.

As to the water-absorbing agent according to the present invention, thesaline flow conductivity (SFC) for a 0.69 mass % physiological salinesolution is favorably not less than 50 (10⁻⁷·cm³·s·g⁻¹), more favorablynot less than 70 (10⁻⁷·cm³·s·g⁻¹), most favorably not less than 100(10⁻⁷·cm³·s·g⁻¹). In the case where the SFC is less than 50(10⁻⁷·cm³·s·g⁻¹), there is a possibility that the liquid permeability orliquid diffusibility may be insufficient in the use for water-absorbentstructures. In addition, favorably, the upper limit value of the SFC is500 (10⁻⁷·cm³·s·g⁻¹). The saline flow conductivity (SFC) for a 0.69 mass% physiological saline solution is measured by the below-mentionedmeasurement method.

As to the water-absorbing agent according to the present invention, thecapillary absorption capacity (CSF) showing the capillary suction forcefor a 0.90 mass % physiological saline solution is favorably not lessthan 15 g/g, more favorably not less than 18 g/g, still more favorablynot less than 20 g/g, most favorably not less than 23 g/g. In the casewhere the CSF is less than 15 g/g, there is a possibility that thedryness property or the liquid-retaining ability may be insufficient inthe use as a portion of a water-absorbent structure. The capillaryabsorption capacity (CSF) for a 0.90 mass % physiological salinesolution is measured by the below-mentioned measurement method.

The CSF of the water-absorbing agent according to the present inventionis not a little influenced by the capillary force of the water-absorbingagent. The capillary force p of the water-absorbing agent has a propertyas shown by the following expression.p∝γ·cos θ/Rcwherein:p: Capillary force of the water-absorbing agentγ: Surface tension of the water-absorbing agentθ: Contact angle of the water-absorbing agentRc: Value corresponding to the capillary radius depending on theparticle diameter distribution of the water-absorbing agent.

As can be understood from the above expression, the capillary force pvaries with the surface tension γ of the water-absorbing agent, thecontact angle θ of the water-absorbing agent, and the value Rccorresponding to the capillary radius depending on the particle diameterdistribution of the water-absorbing agent. That is to say, as thesurface tension γ becomes larger, the capillary force p becomes larger.And, as the contact angle θ nears 0, the capillary force p becomeslarger. Therefore, the surface tension γ and contact angle θ of thewater-absorbing agent according to the present invention are favorablyin the following ranges.

As to the water-absorbing agent according to the present invention, itssurface tension is favorably not less than 30 (mN/m), more favorably notless than 50 (mN/m), most favorably not less than 70 (mN/m). In the casewhere the surface tension is less than 30 (mN/m), there is a possibilitynot only that the CSF may be deteriorated, but also that the objectiveperformances cannot be obtained. The surface tension is measured by thebelow-mentioned measurement method.

As to the water-absorbing agent according to the present invention, itscontact angle is favorably not more than 80°, more favorably not morethan 50°, most favorably not more than 30°. In the case where thecontact angle is more than 80°, there is a possibility not only that theCSF may be deteriorated, but also that the objective performances cannotbe obtained. The contact angle is measured by the below-mentionedmeasurement method.

The water-absorbing agent according to the present invention combinesexcellent liquid permeability and capillary suction force. The liquidpermeability and the capillary suction force has a correlation suchthat, if either one of them is enhanced, the other is deteriorated.However, the water-absorbing agent according to the present inventionhas excellent relations as have never been before. That is to say, thewater-absorbing agent according to the present invention, favorably,satisfies the following relational expression:SFC (10⁻⁷·cm³·s·g⁻¹)≧ε−8×CSF (g/g)wherein ε is a constant and ε=260In addition, the water-absorbing agent according to the presentinvention, more favorably, satisfies the aforementioned expression whenε=270 and, most favorably, satisfies the aforementioned expression whenε=280.

That is to say, the water-absorbing agent according to the presentinvention is favorably a particulate water-absorbing agent comprisingthe water-absorbent resin particles (α) wherein the water-absorbentresin particles (α) are further-surface-crosslink-treatedirregular-shaped pulverized particles of a crosslinked polymer of amonomer including acrylic acid and its salt;

wherein the particulate water-absorbing agent satisfiesSFC (10⁻⁷·cm³·s·g⁻¹)≧260−8×CSF (g/g)and has: an SFC in the range of 50 to 500 (10⁻⁷·cm³·s·g⁻¹), amass-average particle diameter (D50) in the range of 234 to 394 μm and alogarithmic standard deviation (σζ) of a particle diameter distributionin the range of 0.25 to 0.45.

More favorably, the aforementioned water-absorbing agent furthercomprises the liquid-permeability-enhancing agent (β), and theliquid-permeability-enhancing agent (β) content is in the range of 0.01to 5 mass parts per 100 mass parts of the water-absorbent resinparticles (α).

Accordingly, the water-absorbing agent according to the presentinvention combines the liquid permeability and the capillary suctionforce and therefore, in diapers, can display excellent liquiddiffusibility and further can decrease the wet-back amount. In addition,the water-absorbing agent according to the present invention can displaythe aforementioned features in diapers having high-concentration cores,particularly, in diapers having core concentrations of not less than 50mass %.

As to the water-absorbing agent according to the present invention, theabsorption capacity under load (AAP) for a 0.9 mass % physiologicalsaline solution under 4.83 kPa in 60 minutes is favorably in the rangeof 15 to 29 g/g, more favorably 20 to 27 g/g.

Although not especially limited, the water content of thewater-absorbing agent according to the present invention is favorably inthe range of 0 to 400 mass %, more favorably 0.01 to 40 mass %, stillmore favorably 0.1 to 10 mass %.

The water-absorbing agent according to the present invention may bethose which have a bulk density of less than 0.40 g/ml or more than 0.90g/ml. However, the bulk density is favorably in the range of 0.40 to0.90 g/mil, more favorably 0.50 to 0.80 g/ml (the method for measuringthe bulk density is specified in JIS K-3362). In the case ofwater-absorbing agents which have a bulk density of less than 0.40 g/mlor more than 0.90 g/ml, there is a possibility that they may be damagedeasily by the process and may accordingly be deteriorated in property.

(5) Process for Production of Water-Absorbent Structure and WaterAbsorption Properties:

The water-absorbing agent according to the present invention can becombined with an appropriate material and thereby formed into thewater-absorbent structure which is, for example, favorable as anabsorbent layer for sanitary materials. Hereinafter, a description ismade about the water-absorbent structure.

The water-absorbent structure refers to a molded composition whichcomprises a water-absorbent resin or water-absorbing agent and anothermaterial and is used for sanitary materials (e.g. disposable diapers,sanitary napkins, incontinent pads, and medical pads) for absorption ofsuch as blood, body fluids, and urine. Examples of the above othermaterial include cellulose fibers. Specific examples of the cellulosefibers include: wood pulp fibers from wood, such as mechanical pulp,chemical pulp, semichemical pulp, and dissolving pulp; and syntheticcellulose fibers, such as rayon and acetate. Favorable cellulose fibersare the wood pulp fibers. These cellulose fibers may partially containsynthetic fibers such as nylon and polyester. When the water-absorbingagent according to the present invention is used as a portion of thewater-absorbent structure, the mass of the water-absorbing agentaccording to the present invention as contained in the water-absorbentstructure is favorably in the range of 20 to 100 mass %. In the casewhere the mass of the water-absorbing agent according to the presentinvention as contained in the water-absorbent structure is smaller than20 mass %, there is a possibility that no sufficient effects can beobtained.

For the purpose of obtaining the water-absorbent structure from thewater-absorbing agent (as obtained by the above process) and thecellulose fibers, for example, publicly known means for obtainingwater-absorbent structures can appropriate be selected from among suchas: a method in which the water-absorbing agent is spread onto paper ormat made of such as the cellulose fibers and is, if necessary,interposed therebetween; and a method in which the cellulose fibers andthe water-absorbing agent are uniformly blended together. A favorablemethod is a method in which the water-absorbing agent and the cellulosefibers are mixed together in a dry manner and then compressed. Thismethod can greatly prevent the water-absorbing agent from falling offfrom the cellulose fibers. The compression is favorably carried outunder heating, and its temperature range is favorably the range of 50 to200° C. In addition, for the purpose of obtaining the water-absorbentstructure, methods as disclosed in JP-A-509591/1997 (Kohyo) andJP-A-290000/1997 (Kokai) are also favorably used.

In the case where used for water-absorbent structures, thewater-absorbing agent according to the present invention is so good inthe balance between the liquid permeability and the capillary suctionforce as to give water-absorbent structures which are very excellent inthat they quickly take liquids in and further in that the amount of theliquids remaining on their surface layers is small.

In addition, because the above water-absorbing agent has these excellentwater absorption properties, this water-absorbing agent can be used aswater-absorbing and water-retaining agents for various purposes. Forexample, this water-absorbing agent can be used for such as:water-absorbing and water-retaining agents for absorbent articles (e.g.disposable diapers, sanitary napkins, incontinent pads, and medicalpads); agricultural and horticultural water-retaining agents (e.g.substitutes for peat moss, soil-modifying-and-improving agents,water-retaining agents, and agents for duration of effects ofagricultural chemicals); water-retaining agents for buildings (e.g.dew-condensation-preventing agents for interior wall materials, cementadditives); release control agents; coldness-retaining agents;disposable portable body warmers; sludge-solidifying agents;freshness-retaining agents for foods; ion-exchange column materials;dehydrating agents for sludge or oil; desiccating agents; andhumidity-adjusting materials. In addition, the water-absorbing agent asobtained in the present invention can be used particularly favorably forsanitary materials for absorption of excrement, urine, or blood, such asdisposable diapers and sanitary napkins.

In the case where the water-absorbent structure is used for sanitarymaterials (e.g. disposable diapers, sanitary napkins, incontinent pads,and medical pads), this water-absorbent structure is used favorably witha constitution including: (a) a liquid-permeable top sheet placed so asto be adjacent to a wearer's body; (b) a liquid-impermeable back sheetplaced so as to be adjacent to the wearer's clothes at a distance fromthe wearer's body; and (c) the water-absorbent structure placed betweenthe top sheet and the back sheet. The water-absorbent structure may bein more than one layer or used along with such as a pulp layer.

In a more favorable constitution, the basis mass of the water-absorbingagent in the water-absorbent structure is favorably in the range of 60to 1,500 g/m², more favorably 100 to 1,000 g/m², still more favorably200 to 800 g/m².

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is more specifically illustrated bythe following Examples of some preferred embodiments in comparison withComparative Examples not according to the present invention. However,the present invention is not limited to these Examples. The performancesof the water-absorbent resin particles or water-absorbing agents weremeasured by the following methods. The following measurement was carriedout under conditions of a room temperature (25° C.) and a humidity of 50RH %.

Incidentally, in cases of water-absorbing agents having been used forend products such as sanitary materials, the water-absorbing agents havealready absorbed moisture. Therefore, the measurement may be carried outafter appropriately separating the water-absorbing agents from the endproducts and then drying the separated water-absorbing agents under areduced pressure at a low temperature (e.g. under not higher than 1 mmHgat 60° C. for 12 hours). In addition, all the water-absorbing agents asused in the Examples and Comparative Examples of the present inventionhad water contents of not higher than 6 mass %.

(1) Absorption Capacity without Load (Absorption Capacity withoutLoad/CRC for 0.90 Mass % Physiological Saline Solution without Load in30 Minutes):

An amount of 0.20 g of water-absorbent resin particles orwater-absorbing agent was uniformly placed and sealed into a bag (85mm×60 mm) made of nonwoven fabric (trade name: Heatron Paper, type:GSP-22, produced by Nangoku Pulp Kogyo Co., Ltd.) and then immersed intoa large excess (usually about 500 ml) of 0.9 mass % physiological salinesolution of room temperature. After 30 minutes, the bag was pulled upand then drained of water by centrifugal force (as disclosed in edanaABSORBENCY II 441.1-99) with a centrifugal separator (produced byKokusan Co., Ltd., centrifugal separator: model H-122) for 3 minutes,and then the mass W1 (g) of the bag was measured. In addition, the sameprocedure as the above was carried out without the water-absorbent resinparticles or water-absorbing agent, and the resultant mass W0 (g) wasmeasured. Then, the absorption capacity (g/g) without load wascalculated from these W1 and W0 in accordance with the followingequation:Absorption capacity (g/g) without load=(W1 (g)−W0 (g))/(mass (g) ofwater-absorbent resin particles or water-absorbing agent)−1

(2) Absorption Capacity Under Load (Absorption Capacity Under Load/AAPfor 0.90 Mass % Physiological Saline Solution Under Load of 4.83 kPa in60 Minutes):

The measurement was carried out with an apparatus as shown in FIG. 1.

A stainless metal gauze 101, which was a screen of 400 meshes (meshopening size: 38 μm), was attached by fusion to a bottom of a plasticsupporting cylinder 100 having an inner diameter of 60 mm. Then, underconditions of a room temperature (20 to 25° C.) and a humidity of 50 RH%, onto the above gauze, there was uniformly spread 0.90 g ofwater-absorbing agent 102, and further thereon, there were mounted apiston 103 and a load 104 in sequence, wherein the piston had an outerdiameter of only a little smaller than 60 mm and made no gap with theinner wall surface of the supporting cylinder, but was not hindered frommoving up and down, and wherein the piston and the load were adjusted sothat a load of 4.83 kPa (0.7 psi) could uniformly be applied to thewater-absorbing agent Then, the mass Wa (g) of the resultant one set ofmeasurement apparatus was measured.

A glass filter plate 106 having a diameter of 90 mm (produced by SogoRikagaku Glass Seisakusho Co., Ltd., pore diameter: 100 to 120 μm) wasmounted inside a Petri dish 105 having a diameter of 150 mm, and then a0.90 mass % physiological saline solution 108 (20 to 25° C.) was addedup to the same level as the top surface of the glass filter plate, onwhich a filter paper 107 having a diameter of 90 mm (produced byADVANTEC Toyo Co., Ltd., trade name: (JIS P 3801, No. 2), thickness:0.26 mm, diameter of captured particles: 5 μm) was then mounted so thatits entire surface would be wetted, and further, an excess of liquid wasremoved.

The above one set of measurement apparatus was mounted on the above wetfilter paper, thereby getting the liquid absorbed under the load. Then,1 hour later, the one set of measurement apparatus was removed by beinglifted to measure its mass Wb (g). Then, the absorption capacity (g/g)under load was calculated from the Wa and Wb in accordance with thefollowing equation:Absorption capacity (g/g) under load=(Wb (g)−Wa (g))/mass ((0.9) g) ofwater-absorbing agent

(3) Mass-Average Particle Diameter (D50) and Logarithmic StandardDeviation (σζ) of Particle Diameter Distribution:

Water-absorbent resin particles or water-absorbing agents wereclassified with JIS standard sieves having mesh opening sizes of such as850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, and 45μm. Then, the percentages R of the residues on these sieves were plottedon a logarithmic probability paper. Therefrom, a particle diametercorresponding to R=50 mass % was read as the mass-average particlediameter (D50). In addition, the logarithmic standard deviation (σζ) ofthe particle diameter distribution is shown by the following equation.The smaller σζ value shows the narrower particle diameter distribution.σζ=0.5×ln(X2/X1)(wherein: X1 is a particle diameter when R=84.1%, and X2 is a particlediameter when R=15.9%)

As to the classification method for measuring the mass-average particlediameter (D50) and the logarithmic standard deviation (σζ) of theparticle diameter distribution, 10.0 g of water-absorbent resinparticles or water-absorbing agent was placed onto JIS standard sieves(having mesh opening sizes of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm,300 μm, 212 μm, 150 μm, and 45 μm) (THE IIDA TESTING SIEVE: diameter=8cm) under conditions of a room temperature (20 to 25° C.) and a humidityof 50 RH %, and then classified with a shaking classifier (IIDA SIEVESHAKER, TYPE: ES-65 type, SER. No. 0501) for 5 minutes.

(4) Saline Flow Conductivity (SFC) for 0.69 Mass % Physiological SalineSolution:

The saline flow conductivity (SFC) for a 0.69 mass % physiologicalsaline solution is a value showing the liquid permeability displayed bythe water-absorbing agent when it is swollen. The larger SFC value showsthe higher liquid permeability.

The following test was carried out according to the saline flowconductivity (SFC) test as described in JP-A-509591/1997 (Kohyo).

An apparatus as shown in FIG. 2 was used, and a water-absorbing agent(0.900 g) as uniformly placed in a receptacle 40 was swollen insynthetic urine (1) under a load of 0.3 psi (2.07 kPa) for 60 minutes,and the gel layer height of the resultant gel 44 was recorded. Next,under the load of 0.3 psi (2.07 kPa), a 0.69 mass % physiological salinesolution 33 was passed through the swollen gel layer from a tank 31under a constant hydrostatic pressure. This SFC test was carried out atroom temperature (20 to 25° C.). The amount of the liquid passingthrough the gel layer was recorded as a function to time with a computerand a balance at twenty seconds' intervals for 10 minutes. The rateF_(s) (t) of the flow passing through the swollen gel 44 (mainly betweenparticles thereof) was determined in a unit of g/s by dividing theincremental mass (g) by the incremental time (s). The time when theconstant hydrostatic pressure and the stable flow rate were obtained wasrepresented by t_(s), and only the data as obtained between t_(s) and 10minutes were used for the flow rate calculation. The F_(s) (t=0) value,namely, the initial rate of the flow passing through the gel layer, wascalculated from the flow rates as obtained between t_(s) and 10 minutes.The F_(s) (t=0) was calculated by extrapolating the results of aleast-squares fit of F_(s) (t) versus time to t=0.Saline flow conductivity for 0.69 mass % physiological salinesolution=(F _(s)(t=0)×L ₀)/(ρ×A×ΔP)=(F _(s)(t=0)×L ₀)/139,506where:F_(s) (t=0): flow rate denoted by g/s;L₀ initial thickness of gel layer denoted by cm;ρ: density of NaCl solution (1.003 g/cm³);A: area of top of gel layer in cell 41 (28.27 cm²);ΔP: hydrostatic pressure applied to gel layer (4,920 dyne/cm²); andthe unit of the SFC value is: (10⁻⁷·cm³·s·g⁻¹).

As to the apparatus as shown in FIG. 2, a glass tube 32 was inserted inthe tank 31, and the lower end of the glass tube 32 was placed so thatthe 0.69 mass % physiological saline solution 33 could be maintained ata height of 5 cm from the bottom of the swollen gel 44 in the cell 41.The 0.69 mass % physiological saline solution 33 in the tank 31 wassupplied to the cell 41 through an L-tube 34 having a cock 35. Areceptacle 48 to collect the passed liquid was placed under the cell 41,and this collecting receptacle 48 was set on a balance 49. The innerdiameter of the cell 41 was 6 cm, and a No. 400 stainless metal gauze(mesh opening size: 38 μm) 42 was set at the bottom thereof. Holes 47sufficient for the liquid to pass through were opened in the lowerportion of a piston 46, and its bottom portion was equipped with awell-permeable glass filter 45 so that the water-absorbing agent or itsswollen gel would not enter the holes 47. The cell 41 was placed on astand to put the cell thereon. The face, contacting with the cell, ofthe stand was set on a stainless metal gauze 43 that did not inhibit theliquid permeation.

The synthetic urine (1) as used was obtained by mixing together thefollowing: 0.25 g of calcium chloride dihydrate; 2.0 g of potassiumchloride; 0.50 g of magnesium chloride hexahydrate; 2.0 g of sodiumsulfate; 0.85 g of ammonium dihydrogenphosphate; 0.15 g of diammoniumhydrogenphosphate; and 994.25 g of pure water.

(5) Capillary Absorption Capacity (CSF) for 0.90 Mass % PhysiologicalSaline Solution:

The CSF is an index showing the capillary suction force of thewater-absorbing agent.

The capillary absorption capacity in the present invention is determinedby measuring the ability of the absorbent structure to absorb a liquidagainst a negative pressure gradient of the water column of 20 cm undera load of 0.06 psi within a predetermined time. While referring to FIG.3, an apparatus and method for measuring the capillary absorptioncapacity are described.

A conduit 3 was connected to a lower portion of a glass filter 2 of 60mm in diameter having a liquid-absorbing surface of a porous glass plate1 (glass filter particle No. #3: Buchner type filter TOP 17G-3 (code no.1175-03) produced by Sogo Rikagaku Glass Seisakusho Co., Ltd.), and thisconduit 3 was connected to an opening as provided to a lower portion ofa liquid storage container 4 of 10 cm in diameter. The porous glassplate of the aforementioned glass filter has an average pore diameter of20 to 30 μm, and can retain water in the porous glass plate by itscapillary force against the negative pressure of the water column evenin a state where a difference of 60 cm between heights of liquidsurfaces is made, so that a state of no introduction of air can be keptA supporting ring 5 was fitted to the glass filter 2 in order to let upand down its height, and the system was filled with a 0.90 mass %physiological saline solution 6, and the liquid storage container wasput on a balance 7. After it had been confirmed that there was no air inthe conduit and under the porous glass plate of the glass filter, thedifference in height between a liquid surface level of the top of the0.90 mass % physiological saline solution 6 in the liquid storagecontainer 4 and a level of the upside of the porous glass plate 1 wasadjusted to 20 cm, and then the glass filter was fixed to a stand 8.

An amount of 0.44 g of specimen to be measured 9 (water-absorbent resinparticles or water-absorbing agent) was quickly dispersed uniformly ontothe glass filter (porous glass plate 1) in the funnel, and furtherthereon a load 10 (0.06 psi) having a diameter of 59 mm was put, andthen, 30 minutes later, there was measured a value (W20) of the 0.90mass % physiological saline solution as absorbed by the specimen to bemeasured 9.

The capillary absorption capacity is determined from the followingequation.Capillary absorption capacity D1 (g/g) of water-absorbent resinparticles or water-absorbing agent at height of 20 cm=absorption amount(W20) (g)/0.44 (g)

(6) Extractable (Water-Extractable) Component Content:

Into a plastic receptacle of 250 ml in capacity having a lid, 184.3 g of0.90 mass % physiological saline solution was weighed out. Then, 1.00 gof water-absorbent resin particles or water-absorbing agent was added tothis aqueous solution, and they were stirred for 16 hours, thereby theextractable component content in the resin was extracted. This extractliquid was filtrated with a filter paper (produced by ADVANTEC Toyo Co.,Ltd., trade name: (JIS P 3801, No. 2), thickness: 0.26 mm, diameter ofcaptured particles: 5 μm), and then 50.0 g of the resultant filtrate wasweighed out and used as a measuring solution.

To begin with, only the 0.90 mass % physiological saline solution wasfirstly titrated with an aqueous 0.1N NaOH solution until the pH reached10, and then the resultant solution was titrated with an aqueous 0.1NHCl solution until the pH reached 2.7, thus obtaining blank titrationamounts ([bNaOH] ml and [bHCl] ml).

The same titration procedure was carried out for the measuring solution,thus obtaining titration amounts ([NaOH] ml and [HCl] ml).

For example, if the water-absorbent resin or water-absorbent resinparticles or water-absorbing agent comprises acrylic acid and its sodiumsalt in known amounts, the extractable component content of thewater-absorbent resin can be calculated from the average molecularweight of the monomers and the titration amounts, as obtained from theabove procedures, in accordance with the following equation. In the caseof unknown amounts, the average molecular weight of the monomers iscalculated from the neutralization degree as determined by thetitration.Extractable component content (mass %)=0.1×(average molecularweight)×184.3×100×([HCl]−[bHCl])/1,000/1.0/50.0Neutralization degree (mol %)=(1−([NaOH]−[bNaOH])/([HCl]−[bHCl]))×100

(7) Surface Tension:

Into a glass beaker of 120 ml, there was weighed out 80 ml of 0.90 mass% physiological saline solution. Then, 1.00 g of water-absorbing agentwas added to this aqueous solution, and then they were mildly stirredfor 5 minutes. After the stirring had been carried out for 1 minute, thesurface tension of the resultant solution was measured by the platemethod. The surface tension of the 0.90 mass % physiological salinesolution to which no water-absorbing agent had been added was 72 (mN/m).

(8) Contact Angle:

A double-coated pressure-sensitive adhesive tape was stuck onto an SUSsheet, and then the water-absorbent resin particles or water-absorbingagent was spread onto this double-coated tape, and then thewater-absorbent resin particles or water-absorbing agent which had notadhered to the double-coated tape was scraped off to prepare a specimensheet of which the surface was covered with the water-absorbent resinparticles or water-absorbing agent. When a 0.90 mass % physiologicalsaline solution was made to contact with the above specimen sheet, thecontact angle was measured by the sessile drop method with a contactangle meter (FACE CA-X model, produced by Kyowa Kaimen Kagaku K.K.)under conditions of 20° C. and 60% RH. The contact angle at 1 secondlater than dropping a liquid drop of the 0.90 mass % physiologicalsaline solution onto the specimen sheet was measured 5 times per onespecimen. Its average value was determined and taken as the contactangle of the water-absorbent resin particles or water-absorbing agent.

Example 1 (1) Polymerization

In a reactor as prepared by lidding a jacketed stainless twin-armkneader of 10 liters in capacity having two sigma-type blades, there wasprepared a reaction liquid by dissolving 9.36 g (0.08 mol %) ofpolyethylene glycol diacrylate into 5,438 g of aqueous solution ofsodium acrylate having a neutralization degree of 71.3 mol % (monomerconcentration: 39 mass %). Next, this reaction liquid was deaeratedunder an atmosphere of nitrogen gas for 30 minutes. Subsequently, 29.34g of 10 mass % aqueous sodium persulfate solution and 24.45 g of 0.1mass % aqueous L-ascorbic acid solution were added thereto under stirredconditions. As a result, polymerization started after about 1 minute.Then, the polymerization was carried out in the range of 20 to 95° C.while the forming gel was pulverized. Then, the resultant crosslinkedhydrogel polymer (1) was taken out after 30 minutes from the start ofthe polymerization.

The crosslinked hydrogel polymer (1) as obtained above was in the formof finely divided pieces having diameters of not larger than about 5 mm.This finely divided crosslinked hydrogel polymer (1) was spread onto ametal gauze of 50 meshes (mesh opening size: 300 μm) and thenhot-air-dried at 180° C. for 50 minutes, thus obtaining awater-absorbent resin (A1) which was of the irregular shape and easy topulverize, such as in the form of particles, a powder, or a particulatedried material agglomerate.

(2) Pulverization and Classification

The resultant water-absorbent resin (A1) was pulverized with a roll milland then further classified with a JIS standard sieve having a meshopening size of 600 μm. Next, particles having passed through the 600 μmin the aforementioned operation were classified with a JIS standardsieve having a mesh opening size of 180 μm, whereby water-absorbentresin particles (B1F) passing through the JIS standard sieve having themesh opening size of 180 μm were removed, thus obtaining water-absorbentresin particles (B1).

(3) Agglomeration of Fine Powder

The water-absorbent resin particles (B1F) having been removed in theabove “(2) Pulverization and classification” were agglomerated accordingto the method of Granulation Example 1 as disclosed in U.S. Pat. No.6,228,930. The resultant agglomerated material was pulverized andclassified by the same procedure as of the aforementioned (2), thusobtaining agglomerated water-absorbent resin particles (B1A). A viewobtained by taking a photograph of these agglomerated water-absorbentresin particles (B1A) is shown in FIG. 4. As seen therein, thewater-absorbent resin particles (B1A) had a porous structure.

(4) Mixing of Fine-Powder-Agglomerated Product

An amount of 90 mass parts of the water-absorbent resin particles (B1)and 10 mass parts of the water-absorbent resin particles (B1A) wereuniformly mixed together to obtain water-absorbent resin particles(B1A10). The CRC of the water-absorbent resin particles (B1A10) was 33.4g/g.

(5) Surface Treatment

An amount of 100 g of the water-absorbent resin particles (B1A10) asobtained from the aforementioned step were mixed with a surface-treatingagent comprising a mixed liquid of 1.0 g of 1,4-butanediol and 4.0 g ofpure water, and then the resultant mixture was heat-treated at 195° C.for 20 minutes. Furthermore, the resultant particles were disintegratedto such a degree that they could pass through a JIS standard sievehaving a mesh opening size of 600 μm. As a result,surface-crosslink-treated water-absorbent resin particles (C1-1A10) wereobtained. The water-absorbent resin particles (C1-1A10) displayed a CRCof 28.3 g/g, an SFC of 50 (10⁻⁷·cm³·s·g⁻¹), and a CSF of 24.1 g/g.

Next, 100 mass parts of the water-absorbent resin particles (C1-1A10)were uniformly mixed with 0.3 mass part of Reolosil QS-20 (hydrophilicamorphous silica produced by Tokuyama Corporation), thus obtaining awater-absorbing agent (D1-1A10). The resultant water-absorbing agent(D1-1A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3. In addition, alogarithmic normal probability paper as used to determine D50 and σζ isalso shown in FIG. 5.

Example 2

An amount of 100 g of the water-absorbent resin particles (B1A10) asobtained from Example 1 were mixed with a surface-treating agentcomprising a mixed liquid of 0.5 g of 1,4-butanediol, 1.0 g of propyleneglycol, and 4.0 g of pure water, and then the resultant mixture washeat-treated at 210° C. for 25 minutes. Furthermore, the resultantparticles were disintegrated to such a degree that they could passthrough a JIS standard sieve having a mesh opening size of 600 μm. As aresult, surface-crosslink-treated water-absorbent resin particles(C1-2A10) were obtained. The water-absorbent resin particles (C1-2A10)displayed a CRC of 28.0 g/g, an SFC of 60 (10⁻⁷·cm³·s·g⁻¹), and a CSF of24.0 g/g.

Next, 100 mass parts of the water-absorbent resin particles (C1-2A10)were uniformly mixed with 0.3 mass part of Reolosil QS-20 (hydrophilicamorphous silica produced by Tokuyama Corporation), thus obtaining awater-absorbing agent (D1-2A10). The resultant water-absorbing agent(D1-2A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 3

An amount of 100 g of the water-absorbent resin particles (B1A10) asobtained from Example 1 were mixed with a surface-treating agentcomprising a mixed liquid of 2.0 g of propylene glycol and 4.0 g of purewater, and then the resultant mixture was heat-treated at 215° C. for 30minutes. Furthermore, the resultant particles were disintegrated to sucha degree that they could pass through a JIS standard sieve having a meshopening size of 600 μm. As a result, surface-crosslink-treatedwater-absorbent resin particles (C1-3A10) were obtained. Thewater-absorbent resin particles (C1-3A10) displayed a CRC of 27.5 g/g,an SFC of 66 (10⁻⁷·cm³·s·g⁻¹), and a CSF of 23.8 g/g.

Next, 100 mass parts of the water-absorbent resin particles (C1-3A10)were uniformly mixed with 0.3 mass part of Reolosil QS-20 (hydrophilicamorphous silica produced by Tokuyama Corporation), thus obtaining awater-absorbing agent (D1-3A10). The resultant water-absorbing agent(D1-3A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 4 (1) Polymerization

In a reactor as prepared by lidding a jacketed stainless twin-armkneader of 10 liters in capacity having two sigma-type blades, there wasprepared a reaction liquid by dissolving 11.7 g (0.10 mol %) ofpolyethylene glycol diacrylate into 5,438 g of aqueous solution ofsodium acrylate having a neutralization degree of 71.3 mol % (monomerconcentration: 39 mass %). Next, this reaction liquid was deaeratedunder an atmosphere of nitrogen gas for 30 minutes. Subsequently, 29.34g of 10 mass % aqueous sodium persulfate solution and 24.45 g of 0.1mass % aqueous L-ascorbic acid solution were added thereto under stirredconditions. As a result, polymerization started after about 1 minute.Then, the polymerization was carried out in the range of 20 to 95° C.while the forming gel was pulverized. Then, the resultant crosslinkedhydrogel polymer (4) was taken out after 30 minutes from the start ofthe polymerization.

The crosslinked hydrogel polymer (4) as obtained above was in the formof finely divided pieces having diameters of not larger than about 5 mm.This finely divided crosslinked hydrogel polymer (4) was spread onto ametal gauze of 50 meshes (mesh opening size: 300 μm) and thenhot-air-dried at 180° C. for 50 minutes, thus obtaining awater-absorbent resin (A4) which was of the irregular shape and easy topulverize, such as in the form of particles, a powder, or a particulatedried material agglomerate.

(2) Pulverization and Classification

The resultant water-absorbent resin (A4) was pulverized with a roll milland then further classified with a JIS standard sieve having a meshopening size of 600 μm. Next, particles having passed through the 600 μmin the aforementioned operation were classified with a JIS standardsieve having a mesh opening size of 180 μm, whereby water-absorbentresin particles (B4F) passing through the JIS standard sieve having themesh opening size of 180 μm were removed, thus obtaining water-absorbentresin particles (B4).

(3) Agglomeration of Fine Powder

The water-absorbent resin particles (B4F) having been removed in theabove “(2) Pulverization and classification” were agglomerated in thesame way as of Example 1-(3). The resultant agglomerated material waspulverized and classified by the same procedure as of the aforementionedExample 1-(2), thus obtaining agglomerated water-absorbent resinparticles (B4A).

(4) Mixing of Fine-Powder-Agglomerated Product

An amount of 90 mass parts of the water-absorbent resin particles (B4)and 10 mass parts of the water-absorbent resin particles (B4A) wereuniformly mixed together to obtain water-absorbent resin particles(B4A10). The CRC of the water-absorbent resin particles (B4A10) was 31.8g/g.

(5) Surface Treatment

An amount of 100 g of the water-absorbent resin particles (B4A10) asobtained from the aforementioned step were mixed with a surface-treatingagent comprising a mixed liquid of 0.3 g of 1,4-butanediol, 1.5 g ofpropylene glycol, and 3.0 g of pure water, and then the resultantmixture was heat-treated at 220° C. for 25 minutes. Furthermore, theresultant particles were disintegrated to such a degree that they couldpass through a JIS standard sieve having a mesh opening size of 600 μm.As a result, surface-crosslink-treated water-absorbent resin particles(C4-4A10) were obtained. The water-absorbent resin particles (C4-4A10)displayed a CRC of 27.0 g/g, an SFC of 70 (10⁻⁷·cm³·s·g⁻¹), and a CSF of23.1 g/g.

Next, 100 mass parts of the water-absorbent resin particles (C4-4A10)were uniformly mixed with 0.3 mass part of Reolosil QS-20 (hydrophilicamorphous silica produced by Tokuyama Corporation), thus obtaining awater-absorbing agent (D4-4A10). The resultant water-absorbing agent(D4-4A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 5

An amount of 100 g of the water-absorbent resin particles (B4A10) asobtained from Example 4 were mixed with a surface-treating agentcomprising a mixed liquid of 0.4 g of 1,4-butanediol, 0.6 g of propyleneglycol, and 3.0 g of pure water, and then the resultant mixture washeat-treated at 210° C. for 30 minutes. Furthermore, the resultantparticles were disintegrated to such a degree that they could passthrough a JIS standard sieve having a mesh opening size of 600 μm. As aresult, surface-crosslink-treated water-absorbent resin particles(C4-5A10) were obtained. The water-absorbent resin particles (C4-5A10)displayed a CRC of 26.0 g/g, an SFC of 78 (10⁻⁷·cm³·s·g⁻¹), and a CSF of22.1 g/g.

Next, 100 mass parts of the water-absorbent resin particles (C4-5A10)were uniformly mixed with 0.3 mass part of Reolosil QS-20 (hydrophilicamorphous silica produced by Tokuyama Corporation), thus obtaining awater-absorbing agent (D4-5A10). The resultant water-absorbing agent(D4-5A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 6

An amount of 100 mass parts of the water-absorbent resin particles(C4-5A10), as obtained from Example 5, were uniformly mixed with 0.2mass part of Reolosil QS-20 (hydrophilic amorphous silica produced byTokuyama Corporation), thus obtaining a water-absorbing agent (D4-6A10).The resultant water-absorbing agent (D4-6A10) was also of the shape of apowder. The results of having measured its properties are shown inTables 1, 2 and 3.

Example 7 (1) Polymerization

The water-absorbent resin (A4) was obtained in the same way as ofExample 4.

(2) Pulverization and Classification

The resultant water-absorbent resin (A4) was pulverized with a roll milland then further classified with a JIS standard sieve having a meshopening size of 600 μm. Next, particles having passed through the 600 μmin the aforementioned operation were classified with a JIS standardsieve having a mesh opening size of 150 μm, whereby water-absorbentresin particles (B7F) passing through the JIS standard sieve having themesh opening size of 150 μm were removed, thus obtaining water-absorbentresin particles (B7).

(3) Agglomeration of Fine Powder

The water-absorbent resin particles (B7F) having been removed in theabove “(2) Pulverization and classification” were agglomerated in thesame way as of Example 1-(3). The resultant agglomerated material waspulverized and classified by the same procedure as of the aforementioned(2), thus obtaining agglomerated water-absorbent resin particles (B7A).

(4) Mixing of Fine-Powder-Agglomerated Product

An amount of 90 mass parts of the water-absorbent resin particles (B7)and 10 mass parts of the water-absorbent resin particles (B7A) wereuniformly mixed together to obtain water-absorbent resin particles(B7A10). Similarly, 80 mass parts of the water-absorbent resin particles(B7) and 20 mass parts of the water-absorbent resin particles (B7A) wereuniformly mixed together to obtain water-absorbent resin particles(B7A20). Similarly, 70 mass parts of the water-absorbent resin particles(B7) and 30 mass parts of the water-absorbent resin particles (B7A) wereuniformly mixed together to obtain water-absorbent resin particles(B7A30). The CRC of the water-absorbent resin particles (B7) was 32.1g/g, the CRC of the water-absorbent resin particles (B7A10) was 31.8g/g, the CRC of the water-absorbent resin particles (B7A20) was 31.6g/g, and the CRC of the water-absorbent resin particles (B7A30) was 31.3g/g.

(5) Surface Treatment

An amount of 100 g of the water-absorbent resin particles (B7) asobtained from the aforementioned step were mixed with a surface-treatingagent comprising a mixed liquid of 0.5 g of ethylene carbonate and 4.5 gof pure water, and then the resultant mixture was heat-treated at 200°C. for 30 minutes. Furthermore, the resultant particles weredisintegrated to such a degree that they could pass through a JISstandard sieve having a mesh opening size of 600 μm. As a result,surface-crosslink-treated water-absorbent resin particles were obtained.Next, 100 mass parts of these water-absorbent resin particles wereuniformly mixed with 1.0 mass part of Sipernat 22S (hydrophilicamorphous silica obtained from DEGUSSA Corporation), thus obtaining awater-absorbing agent (D7-7A00). The resultant water-absorbing agent(D7-7A00) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

A water-absorbing agent (D7-7A10) was obtained in the same way as theabove except that the water-absorbent resin particles (B7) were replacedwith the water-absorbent resin particles (B7A10). The resultantwater-absorbing agent (D7-7A10) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

A water-absorbing agent (D7-7A20) was obtained in the same way as theabove except that the water-absorbent resin particles (B7) were replacedwith the water-absorbent resin particles (B7A20). The resultantwater-absorbing agent (D7-7A20) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

A water-absorbing agent (D7-7A30) was obtained in the same way as theabove except that the water-absorbent resin particles (B7) were replacedwith the water-absorbent resin particles (B7A30). The resultantwater-absorbing agent (D7-7A30) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

Example 8

An amount of 100 mass parts of the water-absorbent resin particles(C4-5A10) and 0.5 mass part of aluminum sulfate tetradeca- tooctadecahydrates (as prepared by a method in which those which had beenobtained from Wako Pure Chemical Industries, Ltd. were finely pulverizedby grinding them down with a mortar) were uniformly mixed together, thusobtaining a water-absorbing agent (D4-8A10). The resultantwater-absorbing agent (D4-8A10) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

Example 9

An amount of 100 mass parts of the water-absorbent resin particles(C4-5A10) and 0.5 mass part of poly(aluminum chloride) (obtained fromKishida Kagaku K.K.) were uniformly mixed together, thus obtaining awater-absorbing agent (D4-9A10). The resultant water-absorbing agent(D4-9A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 10

The same operation as of Example 4 was carried out except that theconditions of the roll mill were adjusted so that the particle diametersof the resultant water-absorbent resin particles could be still finerones. Thereby, surface-crosslink-treated water-absorbent resin particles(C4-10A10) were obtained. An amount of 100 mass parts of thewater-absorbent resin particles (C4-10A10) and 0.5 mass part of LaponiteRD (obtained from Nippon Silica Kogyo K.K.) were uniformly mixedtogether, thus obtaining a water-absorbing agent (D4-10A10). Theresultant water-absorbing agent (D4-10A10) was also of the shape of apowder. The results of having measured its properties are shown inTables 1, 2 and 3.

Example 11

The same operation as of Example 10 was carried out except that theconditions of the roll mill were adjusted so that the particle diametersof the resultant water-absorbent resin particles could be still finerones. Thereby, surface-crosslink-treated water-absorbent resin particles(C4-11A10) were obtained. An amount of 100 mass parts of thewater-absorbent resin particles (C4-11A10) and 0.5 mass part of Kyowaad700 (obtained from Kyowa Kagaku Kogyo K.K.) were uniformly mixedtogether, thus obtaining a water-absorbing agent (D4-11A10). Theresultant water-absorbing agent (D4-11A10) was also of the shape of apowder. The results of having measured its properties are shown inTables 1, 2 and 3.

Example 12

The same operation as of Example 11 was carried out except that theconditions of the roll mill were adjusted so that the particle diametersof the resultant water-absorbent resin particles could be still finerones. Thereby, surface-crosslink-treated water-absorbent resin particles(C4-12A10) were obtained. An amount of 100 mass parts of thewater-absorbent resin particles (C4-12A10) and 0.7 mass part of alumina(0.5 μm, obtained from Kanto Chemical Co., Inc.) were uniformly mixedtogether, thus obtaining a water-absorbing agent (D4-12A10). Theresultant water-absorbing agent (D4-12A10) was also of the shape of apowder. The results of having measured its properties are shown inTables 1, 2 and 3.

Example 13

A glass container of 6 cm in diameter and 11 cm in height was chargedwith 30 g of the water-absorbing agent (D4-5A10) and 10 g of glass beadsof 6 mm in diameter, and then attached to a paint shaker (product No.488 of Toyo Seiki Seisakusho K.K.), and then shaken at 800 cycles/min(CPM) for 10 minutes. Thereafter, the glass beads were removed, thusobtaining a water-absorbing agent (D4-13A10D). The resultantwater-absorbing agent (D4-13A10D) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

Example 14

A water-absorbing agent (D7-14A10D) was obtained in the same way as ofExample 13 except that the water-absorbing agent (D7-7A10) wassubstituted. The resultant water-absorbing agent (D7-14A10D) was also ofthe shape of a powder. The results of having measured its properties areshown in Tables 1, 2 and 3.

Example 15 (1) Polymerization

The water-absorbent resin (A4) was obtained in the same way as ofExample 4.

(2) Pulverization and Classification

The resultant water-absorbent resin (A4) was pulverized with a roll milland then further classified with a JIS standard sieve having a meshopening size of 500 μm. Next, particles having passed through the 500 μmin the aforementioned operation were classified with a JIS standardsieve having a mesh opening size of 150 μm, whereby water-absorbentresin particles (B15F) passing through the JIS standard sieve having themesh opening size of 150 μm were removed, thus obtaining water-absorbentresin particles (B15).

(3) Agglomeration of Fine Powder

The water-absorbent resin particles (B15F) having been removed in theabove “(2) Pulverization and classification” were agglomerated in thesame way as of Example 1-(3). The resultant agglomerated material waspulverized and classified by the same procedure as of the aforementioned(2), thus obtaining agglomerated water-absorbent resin particles (B15A).

(4) Mixing of Fine-Powder-Agglomerated Product

An amount of 90 mass parts of the water-absorbent resin particles (B15)and 10 mass parts of the water-absorbent resin particles (B15A) wereuniformly mixed together to obtain water-absorbent resin particles(B15A10). The CRC of the water-absorbent resin particles (B15A10) was31.7 g/g.

(5) Surface Treatment

An amount of 100 g of the water-absorbent resin particles (B15A10) asobtained from the aforementioned step were mixed with a surface-treatingagent comprising a mixed liquid of 0.4 g of 2-oxazolidinone, 2.0 g ofpropylene glycol, and 4.0 g of pure water, and then the resultantmixture was heat-treated at 185° C. for 30 minutes. Furthermore, theresultant particles were disintegrated to such a degree that they couldpass through a JIS standard sieve having a mesh opening size of 500 μm.As a result, surface-crosslink-treated water-absorbent resin particles(C15-15A10) were obtained. The water-absorbent resin particles(C15-15A10) displayed a CRC of 26.0 g/g, an SFC of 39 (10⁻⁷·cm³·s·g⁻¹),and a CSF of 23.9 g/g.

Next, 100 mass parts of the water-absorbent resin particles (C15-15A10)were uniformly mixed with 1.0 g of magnesium oxide (0.2 μm, obtainedfrom Wako Pure Chemical Industries, Ltd.), thus obtaining awater-absorbing agent (D15-15A10). The resultant water-absorbing agent(D15-15A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 16

The same operation as of Example 4-(1) to (4) was carried out exceptthat the conditions of the roll mill were adjusted so that the particlediameters of the resultant water-absorbent resin particles could bestill coarser ones. Thereby, water-absorbent resin particles (B16A10)were obtained. An amount of 100 g of the water-absorbent resin particles(B16A10) were mixed with a surface-treating agent comprising a mixedliquid of 1.0 g of ethylene carbonate and 4.0 g of pure water, and thenthe resultant mixture was heat-treated at 200° C. for 40 minutes.Furthermore, the resultant particles were disintegrated to such a degreethat they could pass through a JIS standard sieve having a mesh openingsize of 600 μm. As a result, surface-crosslink-treated water-absorbentresin particles (C16-16A10) were obtained. The water-absorbent resinparticles (C16-16A10) displayed a CRC of 23.1 g/g, an SFC of 113(10⁻⁷·cm³·s·g⁻¹), and a CSF of 19.2 g/g.

Next, 100 mass parts of the water-absorbent resin particles (C16-16A10)were uniformly mixed with 1.0 g of bentonite (obtained from KantoChemical Co., Inc.), thus obtaining a water-absorbing agent (D16-16A10).The resultant water-absorbing agent (D16-16A10) was also of the shape ofa powder. The results of having measured its properties are shown inTables 1, 2 and 3.

Example 17

An amount of 100 mass parts of the water-absorbent resin particles(C16-16A10) and 2.0 mass parts of talc (obtained from Kanto ChemicalCo., Inc.) were uniformly mixed together, thus obtaining awater-absorbing agent (D16-17A10). The resultant water-absorbing agent(D16-17A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 18

An amount of 100 mass parts of the water-absorbent resin particles(C16-16A10) and 1.0 mass part of glass powder Nisshinbo PFE-301s(obtained from Nisshinbo) were uniformly mixed together, thus obtaininga water-absorbing agent (D16-18A10). The resultant water-absorbing agent(D16-18A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 19

An amount of 100 mass parts of the water-absorbent resin particles(C15-15A10) and 1.5 mass parts of Sipernat 2200 (obtained from DEGUSSACorporation) were uniformly mixed together, thus obtaining awater-absorbing agent (D15-19A10). The resultant water-absorbing agent(D15-19A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 20

An amount of 100 mass parts of the water-absorbent resin particles(C15-15A10) and 0.7 mass part of fuller's earth (obtained from KantoChemical Co., Inc.) were uniformly mixed together, thus obtaining awater-absorbing agent (D15-20A10). The resultant water-absorbing agent(D15-20A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 21

An amount of 100 mass parts of the water-absorbent resin particles(C15-15A10) and 1.0 mass part of kaolin (obtained from Kanto ChemicalCo., Inc.) were uniformly mixed together, thus obtaining awater-absorbing agent (D15-21A10). The resultant water-absorbing agent(D15-21A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 22

An amount of 100 mass parts of the water-absorbent resin particles(C4-5A10), 0.1 mass part of Reolosil QS-20 (hydrophilic amorphous silicaproduced by Tokuyama Corporation), and 0.5 mass part of aluminum sulfatetetradeca- to octadecahydrates (as prepared by a method in which thosewhich had been obtained from Wako Pure Chemical Industries, Ltd. werefinely pulverized by grinding them down with a mortar) were uniformlymixed together, thus obtaining a water-absorbing agent (D4-22A10). Theresultant water-absorbing agent (D4-22A10) was also of the shape of apowder. The results of having measured its properties are shown inTables 1, 2 and 3.

Example 23 (1) Polymerization

In a jacketed stainless reactor of 10 liters in capacity as equippedwith stirring blades, there was prepared a reaction liquid by dissolving9.36 g (0.08 mol %) of polyethylene glycol diacrylate into 5,438 g ofaqueous solution of sodium acrylate having a neutralization degree of71.3 mol % (monomer concentration: 39 mass %). Next, this reactionliquid was deaerated under an atmosphere of nitrogen gas for 30 minutes.Subsequently, 54.5 g of 10 mass % aqueous solution of2,2′-azobis(2-methylpropionamidine)dihydrochloride as a foaming agentprecursor was added to the aqueous monomer solution under stirredconditions. Thereafter, the resultant aqueous solution was stirred at atemperature of 25° C. under a nitrogen gas flow. After about 7 minutesfrom the start of the stirring, the aqueous solution became white turbidto form a white fine particulate solid having an average particlediameter of 9 μm. This fine particulate solid was2,2′-azobis(2-methylpropionamidine)diacrylate as a foaming agent. This2,2′-azobis(2-methylpropionamidine)diacrylate was in a state disperseduniformly in the aqueous monomer solution. At that time (after 10minutes from the start of the stirring), 29.34 g of 10 mass % aqueoussodium persulfate solution and 24.45 g of 0.1 mass % aqueous L-ascorbicacid solution were added to the aqueous monomer solution while it wasstirred. After having been stirred enough, the aqueous monomer solutionwas left stationary. As a result, polymerization started after about 1minute. Thereby a bubble-containing hydrogel was obtained after 20minutes. Then, the formed gel was pulverized and then spread onto ametal gauze of 50 meshes (mesh opening size: 300 μm) and thenhot-air-dried at 180° C. for 50 minutes, thus obtaining awater-absorbent resin (A23) which was of the irregular shape and had aporous structure due to bubbles and was easy to pulverize, such as inthe form of particles, a powder, or a particulate dried materialagglomerate.

(2) Pulverization and Classification

The resultant water-absorbent resin (A23) was pulverized with a rollmill and then further classified with a JIS standard sieve having a meshopening size of 600 μm. Next, particles having passed through the 600 μmin the aforementioned operation were classified with a JIS standardsieve having a mesh opening size of 180 μm, whereby water-absorbentresin particles (B23F) passing through the JIS standard sieve having themesh opening size of 180 μm were removed, thus obtaining water-absorbentresin particles (B23).

(3) Mixing

An amount of 90 mass parts of the water-absorbent resin particles (B1)and 10 mass parts of the water-absorbent resin particles (B23) wereuniformly mixed together to obtain water-absorbent resin particles(B23F10). The CRC of the water-absorbent resin particles (B23F10) was33.6 g/g.

(4) Surface Treatment

An amount of 100 g of the water-absorbent resin particles (B23F10) asobtained from the aforementioned step were mixed with a surface-treatingagent comprising a mixed liquid of 1.0 g of 1,4-butanediol, 2.0 g ofpropylene glycol, and 3.0 g of pure water, and then the resultantmixture was heat-treated at 195° C. for 30 minutes. Furthermore, theresultant particles were disintegrated to such a degree that they couldpass through a JIS standard sieve having a mesh opening size of 600 μm.As a result, surface-crosslink-treated water-absorbent resin particles(C23-23F10) were obtained. The water-absorbent resin particles(C23-23F10) displayed a CRC of 26.3 g/g, an SFC of 70 (10⁻⁷·cm³·s·g⁻¹),and a CSF of 21.7 g/g.

Next, 100 mass parts of the water-absorbent resin particles (C23-23F10)were uniformly mixed with 0.3 mass part of Reolosil QS-20 (hydrophilicamorphous silica produced by Tokuyama Corporation), thus obtaining awater-absorbing agent (D23-23F10). The resultant water-absorbing agent(D23-23F10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 24

An amount of 100 mass parts of the water-absorbent resin particles(C16-16A10) and 1.0 mass part of fine particles of polyethylene wereuniformly mixed together, thus obtaining a water-absorbing agent(D16-24A10). The resultant water-absorbing agent (D16-24A10) was also ofthe shape of a powder. The results of having measured its properties areshown in Tables 1, 2 and 3.

Example 25

An amount of 100 mass parts of the water-absorbent resin particles(C15-15A10) and 1.0 mass part of Sipernat D17 (obtained from DEGUSSACorporation) were uniformly mixed together, thus obtaining awater-absorbing agent (D15-25A10). The resultant water-absorbing agent(D15-25A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 26

An amount of 100 mass parts of the water-absorbent resin particles(C16-16A10) and 0.3 mass part of Aerosil R-972 (hydrophobic amorphoussilica obtained from DEGUSSA Corporation) were uniformly mixed together,thus obtaining a water-absorbing agent (D16-26A10). The resultantwater-absorbing agent (D16-26A10) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

Example 27

An amount of 100 mass parts of the water-absorbent resin particles(C4-5A10) and 1.5 mass parts of polyethyleneimine P-1000 (produced byNippon Shokubai Co., Ltd.) were uniformly mixed together, and then theresultant mixture was dried at 90° C. for 60 minutes. Next, the driedmixture was passed through a sieve having a mesh opening size of 600 μm,thus obtaining a water-absorbing agent (D4-27A10). The resultantwater-absorbing agent (D4-27A10) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

Example 28

An amount of 100 mass parts of the water-absorbent resin particles(C15-15A10) and 3 mass parts of Catiofast PR8106 (produced by BASF) wereuniformly mixed together, and then the resultant mixture was dried at90° C. for 60 minutes. Next, the dried mixture was passed through asieve having a mesh opening size of 500 μm, thus obtaining awater-absorbing agent (D15-28A10). The resultant water-absorbing agent(D15-28A10) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Example 29

An amount of 100 mass parts of the water-absorbent resin particles(C16-16A10) and 2 mass parts of 10 mass % aqueous solution ofpolyamidine (produced by Mitsubishi Chemical Corporation) were uniformlymixed together, and then the resultant mixture was dried at 90° C. for60 minutes. Next, the dried mixture was passed through a sieve having amesh opening size of 600 μm, thus obtaining a water-absorbing agent(D16-29A10). The resultant water-absorbing agent (D16-29A10) was also ofthe shape of a powder. The results of having measured its properties areshown in Tables 1, 2 and 3.

Comparative Example 1 (1) Polymerization

The water-absorbent resin (A4) was obtained in the same way as ofExample 4.

(2) Pulverization and Classification

The resultant water-absorbent resin (A4) was pulverized with a roll milland then further classified with a JIS standard sieve having a meshopening size of 850 μm, thus obtaining a water-absorbent resin, most ofwhich was in the range of not larger than 850 μm. Next, thiswater-absorbent resin was classified with a JIS standard sieve having amesh opening size of 150 μm, whereby water-absorbent resin particles(X1F) passing through the JIS standard sieve having the mesh openingsize of 150 μm were removed, thus obtaining water-absorbent resinparticles (X1).

(3) Surface Treatment

An amount of 100 g of the water-absorbent resin particles (X1) asobtained from the aforementioned step were mixed with a surface-treatingagent comprising a mixed liquid of 0.4 g of 1,4-butanediol, 0.6 g ofpropylene glycol, and 3.0 g of pure water, and then the resultantmixture was heat-treated at 212° C. for 30 minutes. Furthermore, theresultant particles were disintegrated to such a degree that they couldpass through a JIS standard sieve having a mesh opening size of 850 μm.As a result, surface-crosslink-treated water-absorbent resin particles(X1-1A00) were obtained. The water-absorbent resin particles (X1-1A00)displayed a CRC of 26.5 g/g, an SFC of 98 (10⁻⁷·cm³·s·g⁻¹), and a CSF of19.1 g/g.

Next, 100 mass parts of the water-absorbent resin particles (X1-1A00)were uniformly mixed with 0.3 mass part of Reolosil QS-20 (hydrophilicamorphous silica produced by Tokuyama Corporation), thus obtaining acomparative water-absorbing agent (Y1-1A00). The resultant comparativewater-absorbing agent (Y1-1A00) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

Comparative Example 2

An amount of 100 mass parts of the water-absorbent resin particles(X1-1A00), as obtained from Comparative Example 1, and 0.2 mass part ofReolosil QS-20 (hydrophilic amorphous silica produced by TokuyamaCorporation) were uniformly mixed together, thus obtaining a comparativewater-absorbing agent (Y1-2A00), The resultant comparativewater-absorbing agent (Y1-2A00) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

Comparative Example 3

An amount of 100 mass parts of the water-absorbent resin particles(X1-1A00), as obtained from Comparative Example 1, and 0.5 mass part ofpoly(aluminum chloride) (obtained from Kishida Kagaku K.K.) wereuniformly mixed together, thus obtaining a comparative water-absorbingagent (Y1-3A00). The resultant comparative water-absorbing agent(Y1-3A00) was also of the shape of a powder. The results of havingmeasured its properties are shown in Tables 1, 2 and 3.

Comparative Example 4

An amount of 100 mass parts of the water-absorbent resin particles(X1-1A00), as obtained from Comparative Example 1, and 1.0 mass part ofkaolin (obtained from Kanto Chemical Co., Inc.) were uniformly mixedtogether, thus obtaining a comparative water-absorbing agent (Y1-4A00).The resultant comparative water-absorbing agent (Y1-4A00) was also ofthe shape of a powder. The results of having measured its properties areshown in Tables 1, 2 and 3.

Comparative Example 5

An amount of 0.5 mass part of aluminum sulfate tetradeca- tooctadecahydrates (as prepared by a method in which those which had beenobtained from Wako Pure Chemical Industries, Ltd. were finely pulverizedby grinding them down with a mortar) and 4.5 mass parts of pure waterwere mixed together to prepare an aqueous solution, and then thisaqueous solution was uniformly mixed with 100 mass parts of thewater-absorbing agent (C4-5A10), and then the resultant mixture wasdried at 60° C. for 20 minutes and then passed through a JIS standardsieve having a mesh opening size of 850 μm, thus obtaining a comparativewater-absorbing agent (Y5-5A00). The resultant comparativewater-absorbing agent (Y5-5A00) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

Comparative Example 6 (1) Polymerization

In a reactor as prepared by lidding a jacketed stainless twin-armkneader of 10 liters in capacity having two sigma-type blades, there wasprepared a reaction liquid by dissolving 3.82 g (0.033 mol %) ofpolyethylene glycol diacrylate into 5,443 g of aqueous solution ofsodium acrylate having a neutralization degree of 75 mol % (monomerconcentration: 39 mass %). Next, this reaction liquid was deaeratedunder an atmosphere of nitrogen gas for 30 minutes. Subsequently, 29.27g of 10 mass % aqueous sodium persulfate solution and 24.22 g of 0.1mass % aqueous L-ascorbic acid solution were added thereto under stirredconditions. As a result, polymerization started after about 1 minute.Then, the polymerization was carried out in the range of 20 to 95° C.while the forming gel was pulverized. Then, the resultant crosslinkedhydrogel polymer (c6) was taken out after 30 minutes from the start ofthe polymerization.

The crosslinked hydrogel polymer (c6) as obtained above was in the formof finely divided pieces having diameters of not larger than about 5 mm.This finely divided crosslinked hydrogel polymer (c6) was spread onto ametal gauze of 50 meshes (mesh opening size: 300 μm) and thenhot-air-dried at 180° C. for 50 minutes, thus obtaining awater-absorbent resin (V6) which was of the irregular shape and easy topulverize, such as in the form of particles, a powder, or a particulatedried material agglomerate.

(2) Pulverization and Classification

The resultant water-absorbent resin (V6) was pulverized with a roll milland then further classified with a JIS standard sieve having a meshopening size of 600 μm. Next, particles having passed through the 600 μmin the aforementioned operation were classified with a JIS standardsieve having a mesh opening size of 150 μm, whereby water-absorbentresin particles (W6F) passing through the JIS standard sieve having themesh opening size of 150 μm were removed, thus obtaining water-absorbentresin particles (W6).

(3) Surface Treatment

An amount of 100 g of the water-absorbent resin particles (W6) asobtained from the aforementioned step were mixed with a surface-treatingagent comprising a mixed liquid of 0.1 g of ethylene glycol diglycidylether, 1.0 g of propylene glycol, and 3.0 g of pure water, and then theresultant mixture was heat-treated at 195° C. for 30 minutes.Furthermore, the resultant particles were disintegrated to such a degreethat they could pass through a JIS standard sieve having a mesh openingsize of 600 μm. As a result, surface-crosslink-treated water-absorbentresin particles (X6-6A00) were obtained. The water-absorbent resinparticles (X6-6A00) displayed a CRC of 35.2 g/g, an SFC of 2(10⁻⁷·cm³·s·g⁻¹), and a CSF of 28.2 g/g.

Next, 100 mass parts of the water-absorbent resin particles (X6-6A00)were uniformly mixed with 0.3 mass part of Reolosil QS-20 (hydrophilicamorphous silica produced by Tokuyama Corporation), thus obtaining acomparative water-absorbing agent (Y6-6A00). The resultant comparativewater-absorbing agent (Y6-6A00) was also of the shape of a powder. Theresults of having measured its properties are shown in Tables 1, 2 and3.

TABLE 1 Water CRC AAP SFC CSF 260-8* content D50 g/g g/g 10⁻⁷ · cm³ · s· g⁻¹ g/g CSF mass % μm σζ Water- absorbing agent D1-1A10 28.5 21.2 8523.2 74 9.8 315 0.37 D1-2A10 28.0 21.5 106 22.1 83 8.9 315 0.37 D1-3A1027.5 21.9 128 21.1 91 8.1 314 0.37 D4-4A10 27.0 21.5 160 17.8 118 8.3316 0.37 D4-5A10 26.0 21.3 178 15.1 139 7.9 314 0.36 D4-6A10 26.4 22.3136 20.9 93 7.7 315 0.36 D7-7A00 26.4 21.9 138 20.7 94 7.6 303 0.38D7-7A10 26.2 22 137 21.3 90 7.8 302 0.38 D7-7A20 26.2 22 140 21.8 86 7.7298 0.38 D7-7A30 26.1 22.1 138 22.5 80 7.7 297 0.38 D4-8A10 25.9 23.7150 20.4 97 7.9 315 0.36 D4-9A10 26 23.7 137 18.3 114 7.8 314 0.36D4-10A10 26 22 135 21.5 88 7.6 297 0.34 D4-11A10 25.8 23.1 132 22.1 837.5 280 0.35 D4-12A10 26.1 22.4 131 22.9 77 7.7 265 0.34 D4-13A10D 26.122.5 136 20.4 97 7.6 306 0.37 D7-14A10D 26.3 23.3 123 22.3 82 7.5 2940.37 D15-15A10 25.8 22.2 90 21.4 88.8 7.8 324 0.30 D16-16A10 22.8 20.1126 18.7 110 7.7 340 0.37 D16-17A10 22.3 20 124 18.6 111 7.3 337 0.38D16-18A10 23.1 20.2 128 18.3 114 7.4 336 0.38 D15-19A10 25.8 21.2 9221.1 91 7.8 324 0.30 D15-20A10 25.9 23.2 75 23.6 71 7.8 320 0.32D15-21A10 25.9 23.5 73 23.7 70 7.7 328 0.29 D4-22A10 25.9 23.1 161 20100 7.6 314 0.37 D23-23F10 26.1 20.1 153 15 140 7.8 315 0.37 D16-24A1023 20.3 134 11.2 170 7.4 348 0.38 D15-25A10 25.9 21.1 134 10.9 173 7.7322 0.31 D16-26A10 22.5 20 178 9.8 182 7.3 341 0.38 D4-27A10 25.9 21.5157 6.5 208 7.7 356 0.33 D15-28A10 25.6 21.4 119 7.8 198 7.7 334 0.29D16-29A10 25.8 21.1 135 6.2 210 7.6 342 0.38 Comparative water-absorbing agent Y1-1A00 26.4 21.3 183 7.6 199 7.9 441 0.49 Y1-2A00 26.421.9 133 8.9 189 7.8 437 0.51 Y1-3A00 26.2 21.5 151 10.1 179 7.6 4310.49 Y1-4A00 26.3 22.3 119 14.9 141 7.7 429 0.51 Y5-5A00 25.8 21.3 1609.8 182 7.7 432 0.50 Y6-6A00 35.2 19.7 10 22.1 83 15.8 342 0.38

TABLE 2 Not smaller Not Water- than 850-710 710-600 600-500 500-425425-300 300-212 212-150 150-45 larger absorbing 850 μm μm μm μm μm μm μmμm μm than 45 μm agent % % % % % % % % % % D1-1A10 0.0 0.0 0.1 2.4 17.535.6 27.4 13.1 3.8 0.1 D1-2A10 0.0 0.0 0.1 2.3 17.3 35.7 27.5 13.2 3.80.1 D1-3A10 0.0 0.0 0.1 2.0 17.2 36.0 27.5 13.1 4.0 0.1 D4-4A10 0.0 0.00.1 2.3 17.6 35.8 27.1 13.5 3.6 0.0 D4-5A10 0.0 0.0 0.0 1.8 17.4 36.128.0 12.7 3.9 0.1 D4-6A10 0.0 0.0 0.0 1.7 17.6 36.2 27.9 12.5 4.0 0.1D7-7A00 0.0 0.0 0.1 1.8 15.5 33.8 28.9 15.3 4.5 0.1 D7-7A10 0.0 0.0 0.11.7 14.7 34.1 29.1 15.5 4.6 0.2 D7-7A20 0.0 0.0 0.1 1.5 13.9 33.9 30.015.8 4.6 0.2 D7-7A30 0.0 0.0 0.1 1.4 12.9 34.5 30.1 16.1 4.7 0.2 D4-8A100.0 0.0 0.1 1.9 17.8 35.8 27.8 12.8 3.7 0.1 D4-9A10 0.0 0.0 0.1 1.7 17.436.2 28.2 13.2 3.2 0.0 D4-10A10 0.0 0.0 0.0 2.1 8.9 38.0 32.0 14.0 4.70.3 D4-11A10 0.0 0.0 0.1 1.6 7.5 33.0 35.8 16.0 5.6 0.4 D4-12A10 0.0 0.00.1 1.0 6.0 28.0 39.9 18.0 6.5 0.5 D4-13A10D 0.0 0.0 0.0 1.3 16.5 34.529.8 13.4 4.3 0.2 D7-14A10D 0.0 0.0 0.0 1.1 12.3 34.8 30.4 16.3 4.9 0.2D15-15A10 0 0 0 0.3 15.1 45.9 28 9.9 0.7 0.1 D16-16A10 0.0 0.0 0.2 9.816 37.9 22.1 10.3 3.5 0.2 D16-17A10 0.0 0.0 0.2 10.2 15.8 36.5 23.1 10.13.9 0.2

TABLE 3 Not smaller Not than 850-710 710-600 600-500 500-425 425-300300-212 212-150 150-45 larger 850 μm μm μm μm μm μm μm μm μm than 45 μm% % % % % % % % % % Water- absorbing agent D16-18A10 0.0 0.0 0 9.5 14.938.6 22 11 3.8 0.2 D15-19A10 0.0 0.0 0 0.3 15.1 45.9 28 9.9 0.7 0.1D15-20A10 0.0 0.0 0 0.1 15 44.2 28.1 11.3 1.1 0.2 D15-21A10 0.0 0.0 00.3 15.4 47.8 27.6 8.2 0.6 0.1 D4-22A10 0.0 0.0 0 1.7 17.3 36.2 27.912.8 4 0.1 D23-23F10 0.0 0.0 0.1 2.5 17.3 35.4 27.4 13.3 3.9 0.1D16-24A10 0.0 0.0 0.1 12.1 17 36.4 21 10.1 3.2 0.1 D15-25A10 0.0 0.0 00.2 15.1 45.1 28.2 10.2 1.1 0.1 D16-26A10 0.0 0.0 0 10.1 16.2 38.2 21.311.1 2.9 0.2 D4-27A10 0.0 0.0 0 5.9 24.1 39.1 19.6 9.1 2.1 0.1 D15-28A100.0 0.0 0 0.2 17.1 49.2 24.1 8.6 0.7 0.1 D16-29A10 0.0 0.0 0 11.3 16.436.3 22 10.2 3.6 0.2 Comparative water- absorbing agent Y1-1A00 0.1 3.525.2 15 12.5 23.3 11.2 5.2 3.8 0.2 Y1-2A00 0.1 4.2 24.5 15.2 12.4 23.110.8 5.5 4 0.2 Y1-3A00 0.0 2.9 24.3 14.9 11.9 24.3 11.4 6 4.1 0.2Y1-4A00 0.0 2.8 24.1 14.8 11.8 24.3 10.8 6.9 4.3 0.2 Y5-5A00 0.0 3.124.4 15.1 11.6 24.2 10.9 6.8 3.7 0.2 Y6-6A00 0.0 0.0 0.3 10.3 17.7 35.422.1 9.7 4.3 0.2

(As to Water-Absorbing Agents as Obtained from Examples 1 to 23 andComparative Examples 1 to 6):

All the water-absorbing agents as obtained from Examples 1 to 23 of thepresent invention satisfy the relational expression “SFC≧260−8·CSF” ofthe present invention and are excellent in both of the liquidpermeability and the capillary suction force. On the other hand, as tothe comparative water-absorbing agents as obtained from ComparativeExamples 1 to 6, the mass-average particle diameter or the logarithmicstandard deviation of the particle diameter distribution is improper, orthe water-extractable component content is high. Therefore, althoughsome of the comparative water-absorbing agents are excellent in theliquid permeability or the capillary suction force, the comparativewater-absorbing agents are not excellent in both them.

The CSF-SFC plots of the water-absorbing agents 1 to 23 according to thepresent invention and of the comparative water-absorbing agents 1 to 6are shown in FIG. 6.

(As to Water-Absorbing Agents as Obtained from Examples 1 to 5):

Examples 1 to 5 show that the effects of theliquid-permeability-enhancing agent (β) vary with the variation of theCRC. The plots of the CRC and SFC before the addition of theliquid-permeability-enhancing agent (β) (those of thesurface-crosslink-treated water-absorbent resin particles) and thoseafter the addition of the liquid-permeability-enhancing agent (β) (thoseof the water-absorbing agents according to the present invention) areshown in FIG. 7. From this FIG. 7, the effects of theliquid-permeability-enhancing agent (β) can be considered as excellentparticularly when the CRC is less than 29 g/g.

(As to Water-Absorbing Agent as Obtained from Example 7):

From the comparison between the water-absorbing agents as obtained fromthis Example, it can be understood that, as the ratio of theagglomerated water-absorbent resin particles as contained in thewater-absorbent resin particles increases, the capillary suction force(CSF) of the resultant water-absorbing agent becomes larger. Therefore,the water-absorbing agent according to the present invention favorably,comprises the liquid-permeability-enhancing agent (β) and thesurface-treated water-absorbent resin particles containing theagglomerated water-absorbent resin particles. That is to say, it isfavorable that at least a portion of the water-absorbent resin particlesincluded in the water-absorbing agent have a porous structure.

(As to Water-Absorbing Agents as Obtained from Examples 13 and 14):

Even after mechanical damage has been done to the water-absorbing agentsaccording to the present invention, these water-absorbing agents displayexcellent performances. Therefore, these water-absorbing agents areadvantageous also in the case of being commercially produced.

INDUSTRIAL APPLICATION

The water-absorbing agent as obtained in the present invention can beused particularly favorably for sanitary materials for absorption ofexcrement, urine, or blood, such as disposable diapers and sanitarynapkins.

1. A water-absorbing agent, which is a particulate water-absorbing agentcomprising water-absorbent resin particles (α) and aliquid-permeability-enhancing agent (β), wherein said water-absorbentresin particles (α) are prepared by crosslinking with asurface-crosslinking agent the surface of irregular-shaped pulverizedparticles of a crosslinked polymer made from a monomer comprisingacrylic acid and/or its salt; wherein the particulate water-absorbingagent has: a mass-average particle diameter in the range of 234 to 394μm, a logarithmic standard deviation (σζ) of a particle diameterdistribution in the range of 0.25 to 0.45, an absorption capacity of notless than 15 g/g without load, and a water-extractable component contentof not higher than 15 mass %; and further aliquid-permeability-enhancing agent (β) content in the range of 0.01 to5 mass parts per 100 mass parts of the water-absorbent resin particles(α).
 2. A water-absorbing agent according to claim 1, wherein thewater-absorbent resin particles (α) included in the water-absorbingagent include particles having a porous structure.
 3. A water-absorbingagent according to claim 1 or 2, which includes particles havingparticle diameters in the range of 100 to 500 μm in an amount of notsmaller than 80 mass % relative to the water-absorbing agent.
 4. Awater-absorbing agent according to claim 1 or 2, wherein thewater-absorbing agent has a mass ratio (particles having particlediameters of not smaller than 300 μm)/(particles having particlediameters of smaller than 300 μm) in the range of 80/20 to 20/80.
 5. Awater-absorbing agent according to claim 1 or 2, wherein thewater-absorbing agent displays a capillary absorption capacity of notless than 15 g/g for a 0.90 mass % physiological saline solution.
 6. Awater-absorbing agent according to claim 1 or 2, wherein thewater-absorbing agent displays a saline flow conductivity of not lessthan 50 (10⁻⁷·cm³·s·g⁻¹) for a 0.69 mass % physiological salinesolution.
 7. A water-absorbing agent according to claim 1 or 2, whereinthe absorption capacity of the water-absorbing agent without load isless than 29 g/g.
 8. A water-absorbing agent according to claim 1 or 2,wherein the liquid-permeability-enhancing agent (β) includeswater-insoluble hydrophilic inorganic fine particles and/or awater-soluble polyvalent metal salt.
 9. A water-absorbing agentaccording to claim 1, wherein the water-absorbing agent is obtained byforming the surface-crosslinked water-absorbent resin particles andthereafter mixing the surface-crosslinked water-absorbent resinparticles with the liquid-permeability-enhancing agent.
 10. Awater-absorbing agent according to claim 9, wherein theliquid-permeability-enhancing agent is added as an aqueous solution,slurry or powder.
 11. A water-absorbing agent according to claim 1,wherein the liquid-permeability-enhancing agent is mixed with thesurface-crosslinked water-absorbent resin particles in an amountsufficient to increase the saline flow conductivity of thewater-absorbent agent.
 12. A water-absorbing agent according to claim11, wherein the liquid-permeability-enhancing agent is added as a powderto the surface-crosslinked water-absorbent resin particles.